Motor control apparatus

ABSTRACT

In an automatic transmission position control by a motor, it is determined whether the present instant belongs to a starting period, that is, the present instant is immediately after resetting of a control unit or application of power to it. If it is the starting period, an actual shift position that is detected from an output of an output shaft sensor for detecting a rotation position of a motor is set as an instructed shift position. With this measure, even if the control unit is reset for a certain reason while the vehicle is running, the instructed shift position is not changed in association with the resetting. This prevents trouble that the shift position is switched contrary to the intention of the driver, whereby the reliability of a position switching control can be increased.

CROSS REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. application Ser. No.11/404,869, filed Apr. 17, 2006 which in turn is a divisional of U.S.application Ser. No. 10/443,795 (now U.S. Pat. No. 7,084,597), filed May23, 2003, the entire contents of which are hereby incorporated byreference in this application. This application is based on andincorporates herein by reference Japanese Patent Application Nos.2002-161849 filed on Jun. 3, 2002, 2002-175377 filed on Jun. 17, 2002,2002-175378 filed on Jun. 17, 2002, 2002-177738 filed on Jun. 18, 2002,2002-177739 filed on Jun. 18, 2002, 2002-207560 filed on Jul. 16, 2002,and 2002-276521 filed on Sep. 24, 2002.

FIELD OF THE INVENTION

The present invention relates to a motor control apparatus forrotationally driving (rotating) the rotor of a motor by sequentiallyswitching the current supply phase of the motor on the basis of anoutput signal of an encoder.

BACKGROUND OF THE INVENTION

Among brushless motors such as switched reluctance motors the demand forwhich has increased recently because they are inexpensive and simple instructure are ones that incorporate an encoder for outputting a pulsesignal in synchronism with the rotation of the rotor and in which therotor is rotated by sequentially switching the current supply phase bycounting pulses of the output signal of the encoder and detecting therotation position of the rotor on the basis of the count.

First, in this type of motor having an encoder, the rotation amount(i.e., a rotation angle) from a start position of the rotor can merelybe detected on the basis of the count, after starting, of an outputsignal of the encoder. Therefore, the motor cannot be driven normallyunless a corresponding relationship between the rotor rotation positionand the current supply phase is obtained by detecting the absoluterotation position of the rotor by a certain method after the applicationof power.

In this connection, for example, JP-A-2000-69779 discloses a switchedreluctance motor in which current supply is simultaneously effected fortwo phases at the beginning of a starting period and a current supplyphase is determined after a lapse of a prescribed time by using, as areference, a rotation position of the rotor at that time. However, meresimultaneous two-phase current supply at the beginning of a startingperiod not always causes the rotor to rotate to a position correspondingto the two current supply phases; there may occur a case that acorresponding relationship between the rotor rotation position and thecurrent supply phase is not obtained. An other mode is proposed in whichcurrent supply is effected for one phase first and for two phasessimultaneously.

An unstable region that will be caused by two-phase current supply iseliminated by effecting one-phase current supply first. The rotor isrotated to a single stable point by later two-phase current supply and areference position of the rotor is thereby learned. However, producinglower torque than two-phase current supply does, one-phase currentsupply may not be sufficient for the rotor to rotate to a correspondingposition. Therefore, merely effecting one-phase current supply first maynot eliminate an unstable region that will be caused by two-phasecurrent supply, and a reference position of the rotor may still belearned erroneously.

Second, where a position switching control is performed with a motorhaving an encoder, if the rotation direction of the rotor is revered dueto some inexorability while the rotor is rotated to a target position onthe basis of the count of the encoder, the count changing direction ofthe encoder is also reversed. As a result, the current supply phaseswitching order that is determined on the basis of the count of theencoder is also reversed to generate torque that drives the rotor in thereverse direction. Once the rotor starts to rotate in the reversedirection, the reverse rotation is accelerated rather than suppressed.This raises a problem that long time is needed to return the rotationdirection from the reverse direction to the normal direction; the rotormay reach a target position with a long delay or, in the worst case, itmay become impossible to stop the reverse rotation, that is, the motormay become uncontrollable.

Third, where the rotor is rotated to a target position by a feedbackcontrol, the following control is performed. Every time a new targetposition is set, a feedback control is performed in which the rotor isrotated toward the target position by switching the current supply phasesequentially on the basis of the count of the encoder. When the count ofthe encoder has reached a target count that is set as corresponding tothe target position, it is determined that the rotor has reached thetarget position, whereupon the feedback control is finished and therotor is stopped at the target position. In this case, the rotor can bekept at the target position by electromagnetic force by continuing toenergize the winding of the phase corresponding to the target positionafter completion of the feedback control. However, in thisconfiguration, if the rotor is stopped for a long time, the winding ofthe same phase continues to be energized for a long time and hence mayoverheat and burn. To prevent the winding from overheating or burning,the winding is not energized while the rotor is stopped.

However, where the rotor is not energized while it is stopped, there isno electromagnetic force for keeping the rotor at a target position(i.e., a position when the rotor stopped) and hence the rotor maydeviate from the target position. One countermeasure is to provide amechanical stopping and holding mechanism for keeping the rotor at atarget position by spring force or the like. However, even with thismeasure, the rotor may still deviate from the target position due toplay amount in the stopping and holding mechanism, its variations inmanufacture, or the like. If the position of the rotor deviates in aperiod when it should be stopped, a feedback control is restarted from acurrent supply phase that is different from A-phase for which currentsupply should be effected first. In this case, the rotor may not berotated normally to a target position; for example, a loss ofsynchronization occurs at the start of the feedback control to cause afailure of starting or the rotor is rotated away from the targetposition.

If the rotor deviates only slightly while it is stopped and the positionof the rotor stays within a range corresponding to a count of theencoder with which the preceding feedback control finished, currentsupply is effected first for A-phase with which the preceding feedbackcontrol finished. However, a feedback control is performed by using, asa reference, a position of the rotor for current supply. Therefore, evenif the deviation of the rotor is so small that its position stays withina range corresponding to a count of the encoder, a loss ofsynchronization may still occur at the start of a feedback control tocause a failure of starting if the feedback control is started aftercurrent supply is effected for A-phase for which current supply shouldbe effected first and before the rotor is moved to a position forcurrent supply and kept there.

Fourth, the rotation amount (i.e., rotation angle) of the motor isconverted via a rotation transmission system into the manipulatedvariable for a control object (a position switching device). However,play amount due to backlash, clearances, etc. exists between constituentparts of the rotation transmission system. Therefore, even if therotation amount of the motor can be controlled correctly on the basis ofthe count of the encoder, an error corresponding to the play amount inthe rotation transmission system occurs in the manipulated variable forthe control object and hence the manipulated variable for the controlobject cannot be controlled accurately.

Fifth, in rotating the rotor to a target position, the current supplyphase switching is performed on the basis of the count of the encoder.To generate torque for rotating the rotor, the phase of the currentsupply phase need to lead the rotation phase of the rotor. As therotation speed of the rotor increases after a start of driving, thevariation rate of the count of the encoder increases and the currentsupply phase switching becomes faster. However, torque is generatedactually with a delay corresponding to the inductance of the winding ofa current supply phase from a start of current supply of the winding.Therefore, if the rotation speed of the rotor is too high, the rotorrotates by a considerable angle from a start of current supply of thecurrent supply phase winding to actual torque generation, that is, thegeneration of torque of the current supply phase delays from the actualrotation phase of the rotor. In this state, the driving torque decreasesand the rotation speed of the rotor lowers. A requirement of increase inposition switching rate (i.e., increase in the rotation speed of therotor) cannot be satisfied.

One countermeasure against this problem would be setting the phase leadof the current supply phase to a large value in advance. However, if thephase lead of the current supply phase is large at a start of driving(i.e., at starting), the starting torque becomes low and the starting ofthe motor becomes unstable or results in a failure. Further, if thephase lead of the current supply phase is set large and the rotationspeed of the rotor is thereby increased, the rotor tends to pass atarget position due to inertia at an end of driving (occurrence of anovershoot), that is, it is difficult to stop the rotor at the targetposition correctly. An other measure for increasing the stability of theposition switching control would be effecting current supply in such amanner that the current supply phase is selected so that rotor istemporarily stopped and kept there at a start of driving, at an end ofthe driving, and when the target position is changed (or the rotationdirection is reversed). However, if this is done by one-phase currentsupply that produces low position-keeping torque, the rotor vibrates andcannot be stopped completely at each position.

Sixth, the motor is controlled by a microcomputer and the microcomputermay be reset for a certain reason (e.g., a short power break) whilecontrolling the motor. If the microcomputer is reset, target positiondata after the resetting become different from those before theresetting because a RAM for storing target position data is also reset.As such, resetting of the microcomputer may cause a problem that acontrol object is switched contrary to the intention so as to bedirected to an unintended position.

Seventh, where a motor feedback control according to JP-A-2001-271917 isemployed, the current supply phase is switched in synchronism withoutput of pulses of a pulse signal of the encoder. Therefore, if therotation of the rotor is stopped for a certain reason during a feedbackcontrol and the output of the pulse signal from the encoder is alsostopped, the current supply phase switching can no longer be performed.This means a problem that the rotor cannot be rotated to a targetposition.

SUMMARY OF THE INVENTION

An object of the present invention is to improve the motor controlapparatus s having the above drawbacks.

According to a first aspect of the invention, in an initial drive thatis performed after application of power, a corresponding relationshipbetween a count of the output signal of the encoder, a rotation positionof the rotor, and a current supply phase at an end of the initial driveis learned by counting pulses of the output signal of the encoder byeffecting current supply for all phases of the motor around according toa prescribed time schedule. In an ordinary drive that is performed afterthe initial drive, a current supply phase is determined on the basis ofa count of the output signal of the encoder and a learning result thatis obtained at the end of the initial drive. As described above, currentsupply is effected for all the phases around in an initial drive,whereby a rotation position of the rotor necessarily coincides with oneof the current supply phases.

From this time onward, the rotor rotates in synchronism with the currentsupply phase switching and pulse output signals are output from theencoder in synchronism with the rotation of the rotor. Therefore, anangle (i.e., a rotation amount) by which the rotor has rotated actuallyin synchronism with the current supply phase switching until the end ofthe initial drive is recognized by checking an encoder count of theoutput signals of the encoder at the end of the initial drive, whereby acorresponding relationship between the encoder count, a rotationposition of the rotor, and a current supply phase at the end of theinitial drive can be recognized. Learning this correspondingrelationship makes it possible to rotate the motor normally by selectinga correct current supply phase in a subsequent ordinary drive on thebasis of a count of the output signals of the encoder and a learningresult at the end of the initial drive.

According to a second aspect of the invention, in rotating the rotor toa target position on the basis of the encoder count, the variationdirection of the encoder count is monitored and it is determined whetherthe rotation direction of the rotor has reversed from the rotationdirection toward the target position. If reversing is detected, currentsupply for phases with which the rotor is driven in the reversedirection is prohibited. With this measure, even if the rotationdirection of the rotor is reversed due to some inexorability and thevariation direction of the encoder count is thereby reversed while therotor is rotated to a target position on the basis of the encoder count,generation of torque that drives the rotor in the reverse rotationdirection can be prevented. Therefore, the time taken to return therotation direction from the reverse direction to the normal directioncan be shortened. A stable position switching control (positioningcontrol) is enabled.

According to a third aspect of the invention, current supply of themotor is turned off after execution of a feedback control in whichrotational driving on the rotor is started each time the target positionis changed and the rotor is rotated to a new target position bysequentially switching the current supply phase of the motor on thebasis of the count of the output signal of the encoder. Before anotherfeedback control is started from a current supply-off state of themotor, in a prescribed period, a feedback control start positionstopping and holding process in which a current supply phase is selectedon the basis of a current count of the output signal of the encoder andcurrent supply is effected for the selected current supply phase, andthe control means thereafter rotates the rotor to a target position byperforming the feedback control.

As described above, a feedback control start position stopping andholding process is executed immediately before the start of a feedbackcontrol, whereby the rotor is kept at a position where the precedingfeedback control finished. Therefore, even if the position of the rotordeviated in a period when the rotor should be stopped, the rotor can bepositioned at a proper position at the start of a feedback control. As aresult, at the start of the feedback control, the position of the rotorand the current supply phase can reliably be synchronized with eachother from the first current supply phase. It becomes possible toprevent a loss of synchronization and prevent the rotor from rotatingaway from a target position at the start of the feedback control, toallow the rotor to reliably rotate to the target position with a stablefeedback control, and to perform a position switching control(positioning control) that is highly stable and reliable.

According to a fourth aspect of the invention, a play amount of arotation transmission system for converting a rotation amount of themotor into a manipulated variable for a control object is learned. Tolearn a play amount of the rotation transmission system, afirst-direction butting control for rotating the rotor until buttingoccurs at one limit position of a movable range of the control objectand a second-direction butting control for rotating the rotor untilbutting occurs at the other limit position of the movable range areperformed. An increase or decrease of the encoder count from the onelimit position to the other limit position is determined as an actualmeasurement value of the movable range of the control object, and adifference between the actual measurement value of the movable range anda design value of the movable range is learned as a play amount of therotation transmission system. When the rotor is thereafter rotated tothe target position, the target position is set by taking the learnedplay amount of the of the rotation transmission system intoconsideration.

According to a fifth aspect of the invention, the phase lead of thecurrent supply phase with respect to the rotation phase of the rotor iscorrected in accordance with the rotation speed of the rotor. Startingtorque can be increased by decreasing the phase lead of the currentsupply phase at a start of driving, whereby the rotation speed of therotor can be raised quickly. Since the phase lead of the current supplyphase is corrected so as to increase as the rotation speed of the rotorincreases, a synchronized relationship between the generation of torqueof the current supply phase and the actual rotation phase of the rotorcan be maintained even during high-speed rotation, which makes itpossible to rotate the rotor stably at high speed. Superior startingperformance and superior high-speed rotation performance can thus beattained simultaneously.

According to a sixth aspect of the invention, a target position isstored in a nonvolatile memory. At starting, the target position storedin the nonvolatile memory is set as a target position of starting. Withthis measure, even if the controller (e.g., a microcomputer) of themotor is reset for a certain reason, at subsequent starting a targetvalue that is stored in the nonvolatile memory immediately before theresetting is set as a target value of starting. Therefore, target valuedata are not changed in association with the resetting, which prevents acontrol object from being rotated to an unintended position.

According to a seventh aspect of the invention, a first current supplyphase is set on the basis of an encoder count in synchronism with outputof a pulse of an output signal of the encoder during a drive control onthe motor. A second current supply phase is set on the basis of theencoder count in a prescribed cycle until the rotor reaches a targetposition. Every time a first or second current supply phase is set, thea winding of the thus-set current supply phase is energized.

With this configuration, even if the rotation of the rotor is oncestopped for a certain reason during a motor drive control and theencoder stops outputting a pulse output signal, the second currentsupply phase is set in the prescribed cycle on the basis of an encodercount of that instant. Therefore, the current supply phase can beswitched even after the rotor was once stopped, and the rotor can berotated so as to come as close to a target position as possible.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the presentinvention will become more apparent from the following detaileddescription made with reference to the accompanying drawings. In thedrawings:

FIG. 1 is a perspective view of a position switching device according toa first embodiment of the present invention;

FIG. 2 shows the structure of an SR motor;

FIG. 3 is a circuit diagram showing the configuration of a circuit fordriving the SR motor;

FIG. 4 schematically shows the configuration of the entire controlsystem of the position switching device;

FIG. 5 is a plan view showing the structure of a rotary magnet of anencoder;

FIG. 6 is a side view of the encoder;

FIG. 7A is a time chart showing output waveforms of the encoder;

FIG. 7B is a time chart showing a current supply phase switchingpattern;

FIGS. 8 and 9 are flowcharts showing an initial drive routine;

FIG. 10 is a flowchart showing a P-position initial drive routine;

FIG. 11 is a flowchart showing a non-P-position initial drive routine;

FIG. 12 is a time chart showing an exemplary control of a P-positioninitial drive;

FIG. 13 is a time chart showing an exemplary control in a case that aposition switching manipulation is performed during an initial drive;

FIG. 14 is a time chart illustrating an initial positional deviationlearning method in a case that a position switching manipulation isperformed during an initial drive;

FIG. 15 is a flowchart showing an encoder counter routine;

FIG. 16 shows an exemplary count increment ΔN calculation map;

FIG. 17 is a time chart showing a relationship between an instructedshift position, an A-phase signal, a B-phase signal, and an encodercount;

FIG. 18 is a time chart showing an exemplary control on the SR motor;

FIG. 19 is a time chart showing timing of a transition from a feedbackcontrol to a target position stopping and holding process;

FIGS. 20-22 are flowcharts showing a control mode setting routine;

FIG. 23 is a flowchart showing a time-synchronous motor control routine;

FIG. 24 is a flowchart showing a mode-1 routine;

FIG. 25 is a flowchart showing a mode-3 routine;

FIG. 26 is a flowchart showing a mode-4 routine;

FIG. 27 is a flowchart showing a current supply processing routine;

FIG. 28 shows an exemplary conversion table to be used for converting avalue Mptn %12 into a current supply phase in the case of aone/two-phase current supply method;

FIG. 29 shows an exemplary table to be used for setting a current supplyduty ratio in accordance with a battery voltage during a buttingcontrol;

FIG. 30 is a flowchart showing a feedback control routine;

FIG. 31 is a flowchart showing a current supply phase setting routine;

FIG. 32 is a time chart illustrating a current supply process in whichrotation is started from a state corresponding to U and W-phases;

FIG. 33 is a flowchart showing a rotor rotation speed calculationroutine;

FIG. 34 a flowchart showing a speed phase lead correction amount settingroutine;

FIG. 35 illustrates a relationship between an engaging portion of adetent spring and each of a P-position holding recess and anon-P-position holding recess of a detent lever;

FIG. 36 is a flowchart showing a play amount learning routine;

FIG. 37 is an exemplary table to be used for setting a ride correctionvalue ΔVover in accordance with a battery voltage;

FIG. 38 illustrates a relationship between an actual measurement valueΔNact and a design value ΔNd of the movable range of the rotor and playamounts ΔGp and ΔGpn;

FIGS. 39 and 40 are flowcharts showing a P-position-side butting controlroutine;

FIG. 41 is a time chart showing an exemplary P-position-side buttingcontrol;

FIGS. 42 and 43 are flowcharts showing a non-P-position-side buttingcontrol routine;

FIG. 44 is a time chart illustrating exemplary execution timing of aP-position-side butting control and a non-P-position-side buttingcontrol;

FIGS. 45 and 46 are flowcharts showing a target count setting routine;

FIG. 47 is a flowchart showing an instructed shift position settingroutine according to the first embodiment;

FIG. 48 is a time chart showing an exemplary control that is performedwhen the CPU is reset in a state that the vehicle is being driven withan instructed shift position sft set to the non-P-position;

FIG. 49 is a flowchart showing an instructed shift position settingroutine according to a second embodiment of the invention;

FIG. 50 is a flowchart showing the first half of a P-position-sidebutting control routine;

FIG. 51 is a flowchart showing a Z-phase correction routine according toa third embodiment of the invention;

FIG. 52 is a time chart illustrating a Z-phase correction;

FIG. 53 is a flowchart showing a current supply method setting routineaccording to a fourth embodiment of the invention;

FIG. 54 shows an exemplary table to be used for converting a value Mptn%12 into a current supply phase in the case of a one-phase currentsupply method;

FIG. 55 is a flowchart showing a time-synchronous motor control routineaccording to a fifth embodiment of the invention; and

FIG. 56 is a flowchart showing a mode-1 routine.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT First Embodiment

A first embodiment in which the present invention is applied to aposition switching device of a vehicle will be herein after describedwith reference to FIGS. 1-48.

First, the configuration of a position switching mechanism 11 will bedescribed with reference to FIG. 1. A motor 12 as a drive source of theposition switching mechanism 11 is a switched reluctance motor, forexample, incorporates a speed-reducing mechanism 26 (see FIG. 4), and isequipped with an output shaft sensor 14 for detecting a rotationposition of the output shaft 13 of the speed-reducing mechanism 26. Adetent lever 15 is fixed to the output shaft 13.

An L-shaped parking rod 18 is fixed to the detent lever 15. A conicalbody 19 that is provided at the tip of the parking rod 18 is in contactwith a lock lever 21. The lock lever 21 is moved in the verticaldirection in accordance with the position of the conical body 19 andthereby locks or unlocks a parking gear 20. The parking gear 20 isattached to the output shaft of an automatic transmission 27. If theparking gear 20 is locked by the lock lever 21, the drive wheels of thevehicle are kept in a rotation-prevented state (i.e., a parking state).

On the other hand, a detent spring 23 for keeping the detent lever 15 atthe position of a parking position (herein after abbreviated as“P-position”) or the other range (herein after referred to as“non-P-position”) is fixed to a support base 17. The detent lever 15 iskept at the P-position when an engaging portion 23 a that is provided atthe tip of the detent spring 23 goes into a P-position holding recess 24of the detent lever 15. The detent lever 15 is kept at thenon-P-position when the engaging portion 23 a of the detent spring 23goes into a non-P-position holding recess 25 of the detent lever 15.

In the P-position, the parking rod 18 is moved in such a direction as toapproach the lock lever 21, whereby a thick portion of the conical body19 lifts up the lock lever 21 and a projection 21 a of the lock lever 21goes into a tooth space of the parking gear 20 to lock it. As a result,the output shaft of the automatic transmission 27 (and the drive wheels)is kept in a locked state (i.e., a parking state).

On the other hand, in the non-P-position, the parking rod 18 is moved insuch a direction as to go away from the lock lever 21, whereby the thickportion of the conical body 19 goes out of contact with the lock lever21 and hence the lock lever 21 goes down. As a result, the projection 21a of the lock lever 21 disengages from the parking gear 20 and hence theparking gear 20 is unlocked. The output shaft of the automatictransmission 27 is kept in a rotatable state (i.e., a state that thevehicle is capable of running).

The above output shaft sensor 14 is a rotary sensor (e.g., apotentiometer) for outputting a voltage corresponding to a rotationangle of the output shaft 13 of the speed-reducing mechanism 26 of themotor 12. Whether the current range is the P-position or thenon-P-position can be recognized on the basis of the output voltage ofthe output shaft sensor 14.

Next, the structure of the motor 12 will be described with reference toFIG. 2. In this embodiment, the motor 12 is a switched reluctance motor(herein after abbreviated as “SR motor”). The SR motor 12, in which bothof a stator 31 and a rotor 32 have salient poles, has an advantage thatno permanent magnet is necessary and hence the structure is simple. Theinner circumferential surface of a cylindrical body of the stator 31 isformed with 12 salient poles 31 a, for example, at regular intervals.

On the other hand, the rotor 32 has eight salient poles 32 a, forexample, that project outward and are arranged at regular intervals inthe circumferential direction. As the rotor 32 rotates, each salientpole 32 a of the rotor 32 faces the salient poles 31 a of the stator 31in order with a small gap interposed in between. The 12 salient poles 31a of the stator 31 are wound with a total of six windings 33 of U, V,and W-phases and a total of six windings 34 of U′, V′, and W-phases inorder. It goes without saying that the number of salient poles 31 a ofthe stator 31 and the number of salient poles 32 a of the rotor 32 maybe changed as appropriate.

In this embodiment, the windings 33 and 34 are wound on the 12 salientpoles 31 a of the stator 31 in order ofV-phase→W-phase→U-phase→V-phase→W-phase→U-phase→V′-phase→W-phase→U′-phase→V′-phase→W-phase→U′-phase,for example. As shown in FIG. 3, the six windings 33 of the U, V, andW-phases and the six windings 34 of the U′, V′, and W-phases areconnected to each other so as to form two systems of motor energizingsections 35 and 36. The one motor energizing section 35 is formed byY-connecting the six windings 33 of the U, V, and W-phases (the twowindings 33 of the same phase are connected to each other in series).The other one motor energizing section 36 is formed by Y-connecting thesix windings 34 of the U′, V′, and W-phases (the two windings 34 of thesame phase are connected to each other in series). In the two motorenergizing sections 35 and 36, the windings of the U-phase and theU′-phase are energized at the same time, the windings of the V-phase andthe V′-phase are energized at the same time, and the windings of theW-phase and the W-phase are energized at the same time.

The two motor energizing sections 35 and 36 are driven by separate motordrivers 37 and 38, respectively, with a battery 40 of the vehicle usedas a power source. Providing the two systems of motor energizingsections 35 and 36 and the two systems of motor drivers 37 and 38 inthis manner makes it possible to rotate the SR motor 12 through theother system even if one system fails. Although the circuitconfiguration of each of the motor drivers 37 and 38 is of a unipolardriving type in which a switching element 39 such as a transistor isprovided for each phase, a bipolar driving type circuit configurationmay be employed in which two switching elements are provided for eachphase. It goes without saying that the invention is also applicable to aconfiguration in which only a single-system motor energizing section andonly a single-system motor driver are provided.

The on/off control on the switching elements 39 of the motor drivers 37and 38 is performed by an ECU 41 (control means). As shown in FIG. 4,the ECU 41 and the motor drivers 37 and 38 are incorporated in aposition switching controller 42. Manipulation signals from a P-positionswitch 43 for a manipulation of switching to the P-position and anon-P-position switch 44 for a manipulation of switching to thenon-P-position are input to the position switching controller 42. Arange that has been selected by manipulating the P-position switch 43 orthe non-P-position switch 44 is indicated in a range indication section45 that is provided in an instrument panel (not shown). The P-positionswitch 43 or the non-P-position switch 44 are momentary switches, forexample.

The SR motor 12 is equipped with an encoder 46 for detecting a rotationposition of the rotor 32. A specific structure of the encoder 46, whichis a magnetic rotary encoder, for example, is as follows. As shown inFIGS. 5 and 6, an annular rotary magnet 47 that is magnetized in such amanner that N poles and S poles are arranged alternately in thecircumferential direction with a constant pitch is fixed to a sidesurface of the rotor 32 concentrically. Three magnetism detectingelements 48-50 such as Hall ICs are opposed to the rotary magnet 47. Inthis embodiment, the magnetization pitch of the N poles and the S polesof the rotary magnet 47 is set at 7.5°, which is equal to a rotationangle of the rotor 32 corresponding to single current supply on the SRmotor 12. As described later, when the current supply phase of the SRmotor 12 is switched six times by a one/two-phase current supply method,current supply is effected for all the phases around and the rotor 32and the rotary magnet 47 rotate together by 7.5°×6=45°. The number of Npoles and S poles existing in this rotation angle range 45° of therotary magnet 47 is six in total.

N poles (N′ poles) corresponding to reference rotation positions of therotor 32 and S poles (S′ poles) on both sides of each of those N polesare wider than the other magnetic poles in the radial direction. In thisembodiment, in view of the fact that the rotor 32 and the rotary magnet47 rotate together by 45° while current supply is effected for all thephases of the SR motor 12 around, the wide magnetized portions (N′)corresponding to the reference rotation positions of the rotor 32 areformed with a pitch of 45°. Therefore, a total of eight wide magnetizedportions (N′) corresponding to the reference rotation positions areformed in the entire rotary magnet 47. Alternatively, one a single widemagnetized portion (N′) corresponding to a reference rotation positionmay be formed in the entire rotary magnet 47.

The three magnetism detecting elements 48-50 are disposed so as to havethe following positional relationships with each other and with therotary magnet 47. The magnetism detecting element 48 for outputting anA-phase signal and the magnetism detecting element 49 for outputting aB-phase signal are disposed on the same circle at such positions as tobe able to face both of the narrow magnetized portions (N and S) and thewide magnetized portions (N′ and S′) of the rotary magnet 47. On theother hand, the magnetism detecting element 50 for outputting a Z-phasesignal is disposed outside or inside the narrow magnetized portions (Nand S) of the rotary magnet 47 at such positions as to be able to faceonly the wide magnetized portions (N′ and S′). The interval between thetwo magnetism detecting elements 48 and 49 for outputting an A-phasesignal and a B-phase signal, respectively, is set so that the phasedifference between the A-phase signal and the B-phase signal becomes 90°in terms of an electrical angle (3.75° in terms of a mechanical angle)as shown in FIG. 7A.

The term “electrical angle” means an angle that is obtained when thecycle of occurrence of each of the A-phase signal and the B-phase signalis regarded as 360°. The term “mechanical angle” is an angle that isobtained when the angle of one rotation of the rotor 32 is regarded as360°; that is, the mechanical angle of the phase difference between theA-phase signal and the B-phase signal is an angle by which the rotor 32rotates in a time from a trailing edge (or rising edge) of the A-phasesignal to that of the B-phase signal. The magnetism detecting element 50for outputting the Z-phase signal is disposed so that the phasedifference between the Z-phase signal and the B-phase signal (or A-phasesignal) becomes 0.

The outputs of the respective magnetism detecting elements 48-50 are ata high level “1”, when they face an N pole (or N′ pole) and are at a lowlevel “0” when they face an S pole (or S′pole). The output (Z-phasesignal) of the magnetism detecting element 50 is at a high level “1”when it faces a wide N′ pole corresponding to a reference rotationposition of the rotor 32 and is at a low level when it is located at theother positions.

In this embodiment, the rotor 32 is rotated as the ECU 41 counts both ofrising edges and trailing edges of the A-phase signal and the B-phasesignal according to an encoder counter routine (described later) andswitches the current supply phase of the SR motor 12 on the basis of theencoder count. In this operation, the ECU 41 determines the rotationdirection of the rotor 32 on the basis of the order of occurrence of anA-phase signal pulse and a B-phase signal pulse. The ECU 41 increasesthe encoder count in the case of normal rotation (i.e., rotation in adirection from the P-position to the non-P position), and decreases theencoder count in the case of reverse rotation (i.e., rotation in adirection from the non-P-position to the P-position). With this measure,the correspondence between the encoder count and the rotation positionof the rotor 32 is maintained irrespective of whether the rotor 32 isrotating in the normal direction or the reverse direction. Therefore,the rotor 32 can be rotated in either the normal direction or thereverse direction by detecting the rotation position (i.e., rotationangle) of the rotor 32 on the basis of the encoder count and energizeswindings 33 or 34 of A-phase corresponding to the detected rotationposition.

FIGS. 7A and 7B show output waveforms of the encoder 46 and a currentsupply phase switching pattern in a case that the rotor 32 is rotated inthe reverse direction (i.e., the rotation direction from thenon-P-position to the P-position), respectively. In either of the casethat the rotor 32 is rotating in the reverse direction (i.e., therotation direction from the non-P-position to the P-position) and thecase that the rotor 32 is rotating in the normal direction (i.e., therotation direction from the P-position to the non-P-position), switchingis made between one-phase current supply and two-phase current supplyevery time the rotor 32 rotates by 7.5°. As the rotor 32 rotates by 45°,current supply is effected for all the phases around in order ofU-phase→U and W-phases→W-phase→V and W-phases→V-phase→U and V-phases,for example. Every time the current supply phase is switched, the rotor32 rotates by 7.5° and the magnetic poles of the rotary magnet 47 thatface the A-phase signal magnetism detecting element 48 and the B-phasesignal magnetism detecting element 49 change from an N pole to an S pole(or an N′ pole to an S′ pole) or from an S pole to an N pole (or an S′pole to an N′ pole), whereby the levels of the A-phase signal and theB-phase signal are inverted alternately.

As a result, the encoder count increases (or decreases) by two everytime the rotor rotates by 7.5°. Further, every time current supply iseffected for all the phases around and the rotor rotates by 7.5°, theZ-phase magnetism detecting element 50 faces a wide N′ polecorresponding to a reference rotation position of the rotor 32 and the Zsignal has a high-level value “1.” In this specification, an event thatthe A-phase signal, the B-phase signal, or the Z-phase signal has ahigh-level value “1” may be referred to as “an A-phase signal pulse, aB-phase signal pulse, or a Z-phase signal pulse is output.”

Since the encoder count is stored in the RAM of the ECU 41, the encodercount disappears when the power to the ECU 41 is turned off. Therefore,an encoder count (“0”) in the ECU 41 immediately after power applicationdoes not correspond to an actual rotation position (or a current supplyphase) of the rotor 32. To switch the current supply phase on the basisof the encoder count, it is necessary to establish correspondencebetween an encoder count and an actual rotation position of the rotor32, that is, between an encoder count and a current supply phase, afterpower application.

In this embodiment, the ECU 41 of the position switching controller 42executes an initial drive routine shown in FIGS. 8 and 9 (describedlater). That is, during an initial drive that is performed after powerapplication to the ECU 41, current supply is effected for all the phasesof the SR motor 12 around according to a prescribed time schedule andedges of the A-phase signal and the B-phase signal of the encoder 46 arecounted. A corresponding relationship between an encoder count, arotation position of the rotor 32, and a current supply phase at thetime of completion of the initial drive is learned. In an ordinary drivethat is performed subsequently, the current supply phase is determinedon the basis of the encoder count and a learning result that wasobtained at the end of the initial drive.

The learning during an initial drive is performed in the followingmanner. As shown in FIG. 12, where an initial drive is performed whenpower is applied to the ECU 41 with the P-position set, current supplyis effected for all the phases around according to a prescribed timeschedule in order of W-phase→U and W-phases→U-phase→U andV-phases→V-phase→V and W-phases, for example, and the rotor 32 is drivenin the normal rotation direction (i.e., the rotation direction from theP-position to the non-P-position).

On the other hand, where an initial drive is performed when power isapplied to the ECU 41 with the non-P-position set, current supply iseffected for all the phases around according to a prescribed timeschedule in order of V-phase→U and V-phases→U-phase→U andW-phases→W-phase→V and W-phases, for example, and the rotor 32 is drivenin the reverse rotation direction (i.e., the rotation direction fromthen on-P-position to the P-position).

In the initial drive, a time T1 of one-phase current supply is setshorter than a time T2 of two-phase current supply; for example, T1 andT2 are set to 10 ms and 100 ms, respectively. In one-phase currentsupply which produces low torque, the rotor 32 vibrates even aftersynchronization has been established between the rotation position ofthe rotor 32 and the current supply phase during an initial drive. Inview of this, the time T1 of one-phase current supply is set short sothat switching is made to the next two-phase current supply as soon aspossible. In this manner, vibration of the rotor 32 is stopped quicklyand the output signals of the encoder 46 are stabilized.

If current supply is effected for all the phases around during aninitial drive in the above manner, the rotation position of the rotor 32and the current supply phase necessarily coincide with each other at acertain current supply phase. From this time onward, the rotor 32rotates in synchronism with the current supply phase switching and theA-phase signal and the B-phase signal are output from the encoder 46 insynchronism with the rotation of the rotor 32.

Both of rising edges and trailing edges of the A-phase signal and theB-phase signal of the encoder 46 are counted during the initial drive.Therefore, an angle (i.e., a rotation amount) by which the rotor 32 hasrotated actually in synchronism with the current supply phase switchinguntil the end of the initial drive can be found by checking an encodercount at the end of the initial drive. In this manner, a correspondingrelationship between an encoder count, a rotation position of the rotor32, and a current supply phase at the end of the initial drive isrecognized.

In the example of FIG. 12, in an initial drive, the rotor 32 starts withthe first current supply phase (W-phase). The rotor rotates by 7.5° andthe encoder count increases by two every time the current supply phaseis switched. The encoder count reaches 12 at the end of the initialdrive.

In contrast, if, for example, the rotor 32 is not rotated by the firstthree current supply attempts (W-phase→U and W-phases→U-phase) and isrotated by the fourth and ensuing current supply attempts (only threecurrent supply attempts; U and V-phases→V-phase→V and W-phases) in sucha manner that the rotation position of the rotor 32 and the currentsupply phase are synchronized with each other, the rotor 32 rotates7.5°×3=22.5° until the end of the initial drive and the encoder countreaches 2×3=6. Therefore, an angle (i.e., a rotation amount) by whichthe rotor 32 has rotated actually in synchronism with the current supplyphase switching until the end of the initial drive can be found bychecking an encoder count at the end of the initial drive.

Whereas the last current supply phases of an initial drive are alwaysthe V and W-phases, the final encoder count need not always be 12 andmay be 8 or 4, for example. In an ordinary drive that is performed afterthe end of an initial drive, the current supply phase is determined onthe basis of the encoder count. Therefore, correct current supply phasescannot be selected unless a deviation in encoder count that has occurredin an initial drive is corrected for.

In view of the above, this embodiment makes it possible to selectcorrect current supply phases during an ordinary drive by learning anencoder count at the end of an initial drive as an initial positionaldeviation learned value and correcting the encoder count using theinitial positional deviation learned value during a subsequent ordinarydrive. In this manner, the deviation between the encoder count and thecurrent supply phase (i.e., the rotation position of the rotor 32) atthe end of the initial drive is corrected for and correct current supplyphases can be selected during the ordinary drive.

After the end of the initial drive, as shown in FIG. 12, a feedbackcontrol start position stopping and holding process (described later) isexecuted in which current supply is effected for the same phases as thecurrent supply phases at the end of the initial drive (i.e., V andW-phases) for 10 ms, for example, whereby the rotor 32 is kept at theposition where it existed at the end of the initial drive. Then, afeedback control (described later) is performed, whereby the rotor 32 isrotated to a target position Acnt by switching the current supply phaseon the basis of an encoder count at that instant and an initialpositional deviation learned value. At an instant when the rotationposition (encoder count) of the rotor 32 has reached a position that is0.5° or less, for example, short of the target position Acnt, thecurrent supply phase switching is finished and the rotor 32 is stopped.Then, a target position stopping and holding process (described later)is executed in which current supply is effected for the same phase(s)and the rotor 32 is kept stopped for 50 ms, for example. Then currentsupply is stopped thereafter unless the target position Acnt is changed.

If a position switching request occurs, that is, the P-position switch43 or the non-P-position switch 44 is manipulated, during an initialdrive, a transition is made to an ordinary drive after the nexttwo-phase current supply is effected (in two-phase current supply itselfif a position switching request occurs during that two-phase currentsupply). And a corresponding relationship between an encoder count, arotation position of the rotor 32, and a current supply phase at the endof the two-phase current supply is learned. In a subsequent ordinarydrive, the current supply phase is determined on the basis of a learningresult at the end of the two-phase current supply and the encoder count.

Producing high torque, two-phase current supply can rotate the rotor 32to a position corresponding to the two-phase current supply even ifthere was some deviation from it. Therefore, the probability would behigh that merely performing two-phase current supply once or twiceduring an initial drive establishes synchronization between the rotationposition and the current supply phase. Therefore, if a positionswitching request occurs during an initial drive, it would be proper tomake a transition to an ordinary drive after completion of the next (orcurrent) two-phase current supply. With this measure, a transition to anordinary drive can be made quickly after a corresponding relationshipbetween an encoder count, a rotation position of the rotor 32, and acurrent supply phase is learned.

For example, as shown in a time chart of FIG. 13, if a positionswitching request from the P-position to the non-P-position occursduring a second current supply attempt (U and W-phases) in an initialdrive, the initial drive is finished with the current supply for the Uand W-phases and a transition is made to an ordinary drive and acorresponding relationship between an encoder count, a rotation positionof the rotor 32, and a current supply phase at the end of the currentsupply for the U and W-phases is learned. In this embodiment, even inthe case where a transition is made to an ordinary drive beforecompletion of an initial drive, an encoder count that would be obtainedat the end of the initial drive is estimated with an assumption that theinitial drive has been completed as scheduled and an estimate value isemployed as an initial positional deviation learned value.

For example, in a case that the initial drive is finished after twocurrent supply attempts for the W-phase and the U and W-phases as shownin FIG. 13, that is, the initial drive is finished without performingfour current supply attempts that are supposed to be performedthereafter, it is assumed that the four current supply attempts(U-phase→U and V-phases→V-phase→V and W-phases) have been performed andan initial positional deviation learned value is calculated by adding acount increase (2×4=8) corresponding to a rotation angle of the fourcurrent supply attempts to an encoder count at the end of the currentsupply for the U and W-phases.

In the conventional position switching control, every time theinstructed shift position (target position) is switched from theP-position to the non-P-position or in the opposite direction, afeedback control is performed in which the rotor 32 is rotated to thetarget position by sequentially switching the current supply phase ofthe SR motor 12 on the basis of the encoder count and the current supplyof the SR motor 12 is then turned off.

In this case, after the end of the feedback control, the rotor 12 can bekept at the target position by electromagnetic force by continuing toenergize the windings of A-phase corresponding to the target position.However, in this configuration, if the rotor 32 is stopped for a longtime, the windings of the same phase continue to be energized for a longtime and hence may overheat and burn. To prevent the windings fromoverheating or burning, the windings are not energized while the rotor32 is stopped.

However, if the rotor 32 is not energized while it is stopped, there isno electromagnetic force for keeping the rotor 32 at the target position(i.e., the position at the end of the feedback control) and hence therotor 32 may deviate from the target position. In this embodiment, themechanical stopping and holding mechanism is provided that keeps therotor 32 at a target position by spring force of the detent spring 23.However, even in this case, the rotor 32 may deviate from the targetposition due to play amount in the stopping and holding mechanism,variations in manufacture, etc.

In the conventional position switching control, to start a feedbackcontrol, a first current supply phase is determined by using an encodercount at the end of the preceding feedback control. The feedback controlis started as soon as the first current supply phase is determined.

Therefore, in the conventional position switching control, if theposition of the rotor 32 deviates while it is stopped, a feedbackcontrol is started with a current supply phase that is different fromA-phase for which current supply should be effected first. In this case,the rotor 32 may not be rotated normally to a target position; forexample, a loss of synchronization occurs at the start of the feedbackcontrol to cause a failure of starting or the rotor 32 is rotated awayfrom the target position.

If the rotor 32 deviates only slightly while it is stopped and hence theposition of the rotor 32 stays within a range corresponding to anencoder count at the end of the preceding feedback control, A-phase forwhich current supply is effected first is a current supply phase at theend of the preceding feedback control. In this case, even theconventional position switching control has no problems about the firstcurrent supply phase.

However, a feedback control is performed by using, as a reference, aposition of the rotor 32 for current supply. Therefore, even if thedeviation of the rotor 32 is so small that its position stays within arange corresponding to an encoder count at the end of the precedingfeedback control, a loss of synchronization may still occur at the startof a feedback control to cause a failure of starting if the feedbackcontrol is started after current supply is effected for A-phase forwhich current supply should be effected first and before the rotor 32 ismoved to a position for current supply and kept there.

As a countermeasure against the above problem, in this embodiment, asshown in FIG. 18, to start a feedback control from a state that the SRmotor 12 is not energized, a process (herein after referred to as“feedback control start position stopping and holding process”) in whichcurrent supply phases are selected on the basis of a current encodercount and the rotor 32 is stopped and kept at a stop position bytwo-phase current supply is executed for a prescribed time (e.g., 10ms). The feedback control is then performed to rotate the rotor 32 to atarget position.

With this measure, even if the rotor 32 deviated in a period when itshould be stopped, the positional deviation of the rotor 32 is correctedfor by the feedback control start position stopping and holding processimmediately before a start of a feedback control, whereby a position ofthe rotor 32 at the start of the feedback control can be determinedcorrectly. Therefore, the position of the rotor 32 and the currentsupply phase (i.e., encoder count) can reliably be synchronized witheach other from the first current supply phase at the start of afeedback control. It becomes possible to prevent a loss ofsynchronization and prevent the rotor 32 from rotating away from atarget position at the start of a feedback control, to allow the rotor32 to reliably rotate to a target position with a stable feedbackcontrol, and to perform a position switching control (positioningcontrol) that is highly stable and reliable.

To increase the response speed of the position switching control, thetime that is taken by the rotor 32 to reach a target position is shortedby increasing its rotation speed. As a result, immediately afterreaching a target position, the rotor 32 has not stopped completely butis vibrating. If the current supply is turned off in such a state,inertia may cause the rotor 32 to stop at a position that greatlydeviates from a target position. If the rotor 32 has too large apositional deviation at the end of a feedback control, the positionaldeviation may not be corrected for even if a feedback control startposition stopping and holding process is executed immediately before thenext feedback control is started.

As a countermeasure against this problem, in this embodiment, thecurrent supply of the SR motor 12 is turned off after a process forstopping the rotor 32 at a position where it existed at the end of afeedback control and keeping the rotor 32 there by two-phase currentsupply (herein after referred to as “target position stopping andholding process”) is executed for a prescribed time (e.g., 50 ms) afterthe end of the feedback control. With this measure, the current supplycan be turned off after the rotor 32 has reached a target position andits vibration has stopped, whereby the rotor 32 is prevented fromstopping at a position deviating greatly from the target position due toinertia. This makes it possible to satisfy the requirement of increasein the response speed of the position switching control whilemaintaining high stability and reliability of the position switchingcontrol.

In this embodiment, if the instructed shift position (target position)is changed during a feedback control to necessitate reversing therotation direction of the rotor 32, a process for stopping the rotor 32at a reversing position and keeping it there by two-phase current supply(herein after referred to as “reversing position stopping and holdingprocess”) is executed for a prescribed time (e.g., 50 ms). The feedbackcontrol is then restarted to rotate the rotor 32 to a new targetposition. With this measure, when the instructed shift position (targetposition) is changed during a feedback control, a reversing operationcan be performed stably by stopping the rotor 32 at a reversing positionand keeping the it there. This prevents an event that deviation of areversing position causes a loss of synchronization (i.e., deviation ofcurrent supply phases) and makes it possible to rotate the rotor 32 to anew target position reliably.

In this embodiment, a feedback control is finished and a transition ismade to a target position stopping and holding process when thedifference between the encoder count and a target count that correspondsto a target position has become smaller than or equal to a prescribedvalue (e.g., a count corresponding to A-phase lead of the current supplyphase) during the feedback control. To rotate the rotor 32, it isnecessary to cause the phase of the current supply phase to lead theactual position of the rotor 32 by 2 to 4 counts (3.75° to 15° in termsof the rotation angle of the rotor 32).

Therefore, if a feedback control is finished when the difference betweenthe encoder count and a target count has become equal to, for example, acount corresponding to A-phase lead of the current supply phase, thelast current supply phase of the feedback control should coincide with acurrent supply phase with which to stop the rotor 32 at the targetposition and keep it there. Therefore, after a transition is made to atarget position stopping and holding process, the current supply for thelast current supply phase of the feedback control can be continued; thetransition from the feedback control to the target position stopping andholding process can be made smoothly.

To generate torque for rotating the rotor 32, it is necessary to causethe phase of the current supply phase to lead the rotation phase of therotor 32. As the rotation speed of the rotor 32 increases after a startof a feedback control, the variation rate of the encoder count increasesand the current supply phase switching becomes faster. However, torqueis generated actually with a delay corresponding to the inductance ofthe windings 33 or 34 of a current supply phase from a start of currentsupply of the windings 33 or 34. Therefore, if the rotation speed of therotor 32 is too high, the rotor 32 rotates by a considerable angle froma start of current supply of the windings 33 or 34 of the current supplyphase to actual generation of torque, that is, the generation of torqueof the current supply phase delays from the actual rotation phase of therotor 32. In this state, the driving torque decreases and the rotationspeed of the rotor 32 lowers. A requirement of increase in positionswitching rate (i.e., increase in the rotation speed of the rotor 32)cannot be satisfied.

One countermeasure against this problem would be setting the phase leadof the current supply phase to a large value in advance. However, if thephase lead of the current supply phase is large at a start of a feedbackcontrol (i.e., at starting), the starting torque becomes low and thestarting of the SR motor 12 becomes unstable or results in a failure.

Further, if the phase lead of the current supply phase is set large andthe rotation speed of the rotor 32 is thereby increased, the rotor 32tends to pass a target position due to inertia at an end of a feedbackcontrol (occurrence of an overshoot), that is, it is difficult to stopthe rotor 32 at the target position correctly.

In view of the above, in this embodiment, the phase lead of the currentsupply phase with respect to the rotation phase of the rotor 32 iscorrected in accordance with the rotation speed of the rotor 32 during afeedback control. And when it becomes necessary to decrease the rotationspeed of the rotor 32 (e.g., because the rotor 32 comes close to atarget position), the phase lead of the current supply phase iscorrected in the decreasing direction. Specifically, correcting thephase lead of the current supply phase in the decreasing direction at astart of a feedback control can increase starting torque and therebyraise the rotation speed of the rotor 32 quickly. Correcting the phaselead of the current supply phase in the increasing direction as therotation speed of the rotor 32 increases can maintain the synchronizedrelationship between the generation of torque of the current supplyphase and the actual rotation phase of the rotor 32 even duringhigh-speed rotation, and hence makes it possible to rotate the rotor 32stably at high speed. This makes it possible to attain both of goodstarting performance and good high-speed rotation performance.

When the rotor 32 comes close to the target position afterwards, thephase lead of the current supply phase is corrected in the decreasingdirection, whereby a state that the generation of torque of the currentsupply phase is delayed from the actual rotation phase of the rotor 32is established. In this manner, the driving torque can be decreased ortorque (braking torque) can be generated in the direction opposite tothe rotation direction of the rotor 32, whereby the rotation speed ofthe rotor 32 can be decreased reliably and the rotor 32 can be stoppedat the target position correctly. This makes it possible to perform afeedback control that is superior in starting performance, high-speedrotation performance, and stop performance (deceleration performance).

The rotation amount (i.e., rotation angle) of the rotor 32 is convertedinto the manipulated variable for the position switching mechanism 11(i.e., the slide length of the parking rod 18) via the rotationtransmission system that is composed of the speed-reducing mechanism 26,the output shaft 13, the detent lever 15, etc. and the constituent partsof the rotation transmission system has play amount. For example,backlash exists between the gears of the speed-reducing mechanism 26. Ina structure in which a connecting portion having a non-circularcross-section that is formed at the tip of the rotary shaft of the motor12 is fitted into a fitting hole of the output shaft 13, a certainclearance is needed to facilitate work of fitting the former into thelatter.

Further, as shown in FIG. 35, when the engaging portion 23 a of thedetent spring 23 goes into the P-position holding recess 24 or thenon-P-position holding recess 25 of the detent lever 15, a slight gap(play amount) exists between the engaging portion 23 a and the sidewalls of the holding recess 24 or 25. As exemplified above, play amountsuch as backlash and gaps between parts exist in the rotationtransmission system for converting the rotation amount of the rotor 32into the manipulated variable for the position switching mechanism 11(i.e., the slide length of the parking rod 18). Even if the rotationamount of the rotor 32 can be controlled correctly on the basis of theencoder count, the manipulated variable for the position switchingmechanism 11 has an error corresponding to the play amount in therotation transmission system; the manipulated variable for the positionswitching mechanism 11 cannot be controlled accurately.

In view of the above, this embodiment is provided with a function oflearning an amount of play amount in the rotation transmission system.Specifically, to learn an amount of play amount in the rotationtransmission system, a P-position-side butting control (i.e., afirst-direction butting control) for rotating the rotor 32 until theengaging portion 23 a of the detent spring 23 butts against a side wallof the P-position holding recess 24 as a P-position-side limit positionof the movable range of the position switching mechanism 11 and anon-P-position-side butting control (i.e., a second-direction buttingcontrol) for rotating the rotor 32 until the engaging portion 23 a ofthe detent spring 23 butts against a side wall of the non-P-positionholding recess 25 as a non-P-position-side limit position are performed,whereby an increase or decrease of the encoder count corresponding tothe range from the P-position-side limit position to thenon-P-position-side limit position is determined as an actualmeasurement value of the movable range of the position switchingmechanism 11.

The difference between this actual measurement value of the movablerange and its design value is learned as a play amount of the rotationtransmission system. When the rotor 32 is rotated to a target positionafterwards, the target position is set by taking the learned play amountof the rotation transmission system into consideration. This makes itpossible to set, even if the rotation transmission system has playamount, a target value taking the play amount into consideration and tothereby control the manipulated variable for the position switchingmechanism 11 accurately.

If there is a sufficient time to learn a play amount of the rotationtransmission system from application of power (i.e., turning-on of theignition switch) to the ECU 41 which controls the SR motor 12 to a startof a control on the position switching mechanism 11, a play amount ofthe rotation transmission system may be learned after application ofpower to the ECU 41 and before a start of a control on the positionswitching mechanism 11 by performing a P-position-side butting controland a non-P-position-side butting control successively. However, whereit is necessary to quickly start a control on the position switchingmechanism 11 after application of power to the ECU 41, there may not bea sufficient time to learn a play amount of the rotation transmissionsystem after application of power to the ECU 41.

In view of the above, in this embodiment, after a control on theposition switching mechanism 11 was started without learning a playamount, in a state that the rotor 32 is stopped at the P-position, aP-position-side butting control is performed and an encoder count whenthe engaging portion 23 a is in contact with the side wall of theP-position holding recess 24 is stored in the RAM of the ECU 41. In astate that the rotor 32 is stopped at the non-P-position, anon-P-position-side butting control is performed and an encoder countwhen the engaging portion 23 a is in contact with the side wall of thenon-P-position holding recess 25 is stored in the RAM of the ECU 41. Thedifference between the encoder counts obtained by the P-position-sidebutting control and the non-P-position-side butting control iscalculated as an actual measurement value of the movable range of theposition switching mechanism 11. The difference between the actualmeasurement value of the movable range and its design value is learnedas a play amount.

With this measure, even if there was not a sufficient time to learn aplay amount of the rotation transmission system from application ofpower to the ECU 41 to a start of a control on the position switchingmechanism 11 and hence a control on the position switching mechanism 11was started without learning a play amount of the rotation transmissionsystem, a play amount can be learned by performing butting controls whenthe rotor 32 is stopped at the P-position and the non-P-position. Inthis case, the same control as in the conventional case that does nottake a play amount of the rotation transmission system intoconsideration may be performed before completion of learning of a playamount. Alternatively, the control object may be controlled by using apreset, average play amount or a storage value of a learned play amountof the preceding learning. In the following description, the abbreviatedterm “butting control” means either of a P-position-side butting controland a non-P-position-side butting control.

In this embodiment, when a butting control is performed, the torque ofthe SR motor 12 is made lower than in an ordinary drive by making thecurrent supply duty ratio (i.e., current supply factor) of the SR motor12 smaller than in an ordinary drive. The torque of the SR motor 12 isset so strong that the engaging portion 23 a of the detent spring 23 canreliably go over the projection between the two holding recesses 24 and25 of the detent lever 15 at the time of position switching. However, inthe butting control, the torque of the SR motor 12 causes the engagingportion 23 a of the detent spring 23 to butt against the holding recess24 or 25 of the detent lever 15. Therefore, if the torque of the SRmotor 12 is too strong, the parts of the rotation transmission systemsuch as the engaging portion 23 a of the detent spring 23 may bedeformed or damaged gradually to lower the durability and reliability asthe number of times of butting controls increases.

As a countermeasure against this problem, the torque of the SR motor 12is reduced during a butting control, whereby the force of butting of theengaging portion 23 a of the detent spring 23 against the side wall ofthe holding recess 24 or 25 can be weakened. This makes it possible toprevent butting controls from deforming or damaging the parts of therotation transmission system such as the engaging portion 23 a of thedetent spring 23 and to thereby secure high durability and reliability.In the butting control of this embodiment, the engaging portion 23 a ofthe detent spring 23 need not go over the projection between the twoholding recesses 24 and 25 of the detent lever 15. Therefore, even ifthe torque of the SR motor 12 is decreased during a butting control, thebutting control can be performed normally as long as the torque ishigher than a minimum value necessary to cause the engaging portion 23 aof the detent spring 23 to butt against the side wall of each of theholding recesses 24 and 25.

In this embodiment, during a butting control, the phase lead of thecurrent supply phase is corrected so as to decrease the rotation speedof the rotor 32. If the rotation speed of the rotor 32 is too highduring butting controls, the engaging portion 23 a of the detent spring23 collides with the side walls of the holding recesses 24 and 25 of thedetent lever 15 at high speed and resulting impact gradually deforms ordamages the parts of the rotation transmission system such as theengaging portion 23 a of the detent spring 23. Decreasing the rotationspeed of the rotor 32 during butting controls lowers the speed at whichthe engaging portion 23 a of the detent spring 23 collides with the sidewalls of the holding recesses 24 and 25 of the detent lever 15 duringbutting controls and hence can weaken resulting impact. This makes itpossible to prevent butting controls from deforming or damaging theparts of the rotation transmission system such as the engaging portion23 a of the detent spring 23 and to thereby secure high durability andreliability.

In this embodiment, during a butting control, an angle (i.e., a ridecorrection value) by which the engaging portion 23 a of the detentspring 23 slightly rides the side wall of the holding recess 24 or 25 ofthe detent lever 15 in a state that the engaging portion 23 a is incontact with the side wall is inferred on the basis of the batteryvoltage as a power source voltage for the SR motor 12. And an actualmeasurement value of the movable range of the position switchingmechanism 11 is corrected by the inferred value. The angle (i.e., ridecorrection value) by which the engaging portion 23 a of the detentspring 23 rides the side wall of the holding recess 24 or 25 of thedetent lever 15 during a butting control increases as the torque of theSR motor 12 increases. In general, the torque of the SR motor 12 varieswith the power source voltage therefor (i.e., battery voltage) and hencea correlation exists between the power source voltage and the torque ofthe SR motor 12. The power source voltage can be used as replacementinformation for the torque of the SR motor 12. Therefore, an actualmeasurement value of the movable range of the position switchingmechanism 11 can be determined accurately by inferring an angle (i.e., aride correction value) by which the engaging portion 23 a of the detentspring 23 rides the side wall of the holding recess 24 or 25 of thedetent lever 15 during a butting control on the basis of the powersource voltage (i.e., battery voltage) as replacement information forthe torque of the SR motor 12 and correcting an actual measurement valueof the movable range of the position switching mechanism 11 by theinferred value.

In this embodiment, a butting control is finished when the encoder countcomes not to vary in the butting control with a determination that theengaging portion 23 a of the detent spring 23 has butted against theside wall of the holding recess 24 or 25 of the detent lever 15. Thismakes it possible to determine a correct encoder count of a state thatthe engaging portion 23 a of the detent spring 23 is in contact with theside wall of the holding recess 24 or 25 of the detent lever 15 and tothereby determine, accurately, an actual measurement value of themovable range of the position switching mechanism 11. The above measurecan also shorten the time during which the engaging portion 23 a of thedetent spring 23 is kept in contact with the side wall of the holdingrecess 24 or 25 of the detent lever 15, which can further reduce thedegree of deterioration of the parts of the rotation transmission systemsuch as the engaging portion 23 a of the detent spring 23 due to buttingcontrols.

In this embodiment, a butting control is finished when it is determinedthat the engaging portion 23 a of the detent spring 23 does not buttagainst the side wall of the holding recess 24 or 25 of the detent lever15 even if the butting control has been performed for a prescribed timeor more. If the engaging portion 23 a of the detent spring 23 does notbutt against the side wall of the holding recess 24 or 25 of the detentlever 15 even if a butting control has been performed for a long time, acertain system abnormality such as a failure of the SR motor 12 or theencoder 46 should have occurred. Forcibly finishing the butting controlin such a case is a fail-safe measure.

Incidentally, there may occur an event that the CPU of the ECU 41 whichcontrols the SR motor 12 is reset for a certain reason (e.g., a shortpower break). Once the CPU of the ECU 41 is reset, the RAM which storesdata of an instructed shift position (target position) is also reset. Aninstructed shift position after the RAM resetting is different from thatbefore the RAM resetting. For example, if the CPU of the ECU 41 whichcontrols the SR motor 12 is reset for a certain reason while the vehicleis driven with the instructed shift position being the non-P-position,the instructed shift position data stored in the RAM is initialized to avalue corresponding to the P-position (indicated by a broken line inFIG. 48). Therefore, if a position switching control is performed on thebasis of an instructed shift position stored in the RAM when the CPU ofthe ECU 41 has been reset during a drive, trouble occurs that the shiftposition is switched contrary to the intention of the driver.

To solve this problem, in this embodiment, with attention paid to theoutput shaft sensor 14 which detects a rotation position of the outputshaft 13 of the SR motor 12, an actual shift position that is detectedby the output shaft sensor 14 at starting (e.g., after resetting of theCPU of the ECU 41 or after power application to it) is set as aninstructed shift position of starting. With this measure, even if theCPU of the ECU 41 is reset for a certain reason during a drive, theinstructed shift position is not changed in association with theresetting. This prevents trouble that the shift position is switchedcontrary to the intention of the driver; the reliability of the positionswitching control can be increased. If the rotation direction of therotor 32 is revered due to some inexorability during a feedback control,the variation direction of the encoder count is also reversed. As aresult, the current supply phase switching order that is determined onthe basis of the encoder count is also reversed to generate torque thatdrives the rotor 32 in the reverse direction. Once the rotor starts torotate in the reverse direction, the reverse rotation is acceleratedrather than suppressed. This raises a problem that long time is neededto return the rotation direction from the reverse direction to thenormal direction; the rotor may reach a target position with a longdelay or, in the worst case, it may become impossible to stop thereverse rotation, that is, the SR motor 12 may become uncontrollable.

As a countermeasure against this problem, in this embodiment, during afeedback control, the variation direction of the encoder count ismonitored and it is determined whether the rotation direction of therotor 32 has reversed from a direction toward a target position. Ifreversing of the rotation direction is detected, current supply forphases with which the rotor 32 is driven in the reverse direction isprohibited and current supply is effected for A-phase for which currentsupply was effected immediately before the reversing. Effecting currentsupply for the phase for which current supply was effected immediatelybefore the reversing can generate braking torque that suppresses thereverse rotation of the rotor 32. The braking torque can suppress thereverse rotation quickly, whereby the rotation direction of the rotor 32can quickly be returned from the reverse direction to the normaldirection.

The above position switching is performed by the ECU 41 of the positionswitching controller 42 according to the following routines, each ofwhich will be described below in detail.

[Initial Drive]

An initial drive is performed according to an initial drive routineshown in FIGS. 8 and 9. This routine is executed in a prescribed cycle(e.g., every 1 ms) until an end of the initial drive immediately afterapplication of power to the ECU 41 (i.e., immediately after manipulationof the ignition switch from the OFF position to the ACC position).

Upon activation of this routine, at step 101, it is determined whetheran open-loop control execution flag Xopen is “on” or a recovery processexecution flag Xrcv is “on.” The open-loop control execution flag Xopenis a flag indicating whether an open-loop control (fail-safe process) tobe executed when the encoder 46 or the SR motor 12 has failed is beingexecuted. The open-loop control execution flag Xopen being “on” meansthat the open-loop control is being executed. The recovery processexecution flag Xrcv is a flag indicating whether a recovery process(open-loop control) to be executed temporarily upon occurrence of atemporal operation abnormality has occurred is being executed. Therecovery process execution flag Xrcv being “on” means that the recoveryprocess is being executed.

If the determination result at step 101 is “yes,” this routine isfinished without executing the remaining steps. If the determinationresult at step 101 is “no,” the routine proceeds to step 102, where itis determined whether a prescribed time (e.g., 100 ms) that is a waitingtime from initialization of the ECU 41 to stabilization of the outputvoltage of the output shaft sensor 14 has elapsed. When the prescribedtime has elapsed from the initialization of the ECU 41, the routineproceeds to step 103, where an output voltage of the output shaft sensor14 is read. Whether the current range is the P-position or thenon-P-position is determined on the basis of whether the output voltageis lower than or equal to a range determination value. If the currentrange is the P-position, the routine proceeds to step 104, where a rangedetermination flag Xnp is set to “0” that means the P-position. If thecurrent range is the non-P-position, the routine proceeds to step 105,where the range determination flag Xnp is set to “1” that means thenon-P-position.

Then, the routine proceeds to step 106, where it is determined whetherthe range determination flag Xnp is “0” (P-position). If the rangedetermination flag Xnp is “0” (P-position), the routine proceeds to step107, where a P-position initial drive routine shown in FIG. 10 isexecuted. If the range determination flag Xnp is “1” (non-P-position),the routine proceeds to step 108, where a non-P-position initial driveroutine shown in FIG. 11 is executed.

If the P-position initial drive routine of FIG. 10 is activated at step107, it is determined at steps 201-206 which of “0” to “5” the countCASE of a current-supply counter that counts the current-supply duringan initial drive is equal to. The count CASE of the current-supplycounter has an initial value “0” that is set in the initializationprocessing and is incremented by one for each current supply (step 114in FIG. 9). A current supply phase and a current supply time T are setin the following manner in accordance with a result of the determinationof the count CASE.

If CASE=0 (first current supply), the routine proceeds to step 207,where W-phase current supply is selected and the current supply time Tis set to T1 (e.g., 10 ms).

If CASE=1 (second current supply), the routine proceeds to step 208,where U/W-phase current supply is selected and the current supply time Tis set to T2 (e.g., 100 ms).

If CASE=2 (third current supply), the routine proceeds to step 209,where U-phase current supply is selected and the current supply time Tis set to T1 (e.g., 10 ms).

If CASE=3 (fourth current supply), the routine proceeds to step 210,where U/V-phase current supply is selected and the current supply time Tis set to T2 (e.g., 100 ms).

If CASE=4 (fifth current supply), the routine proceeds to step 211,where V-phase current supply is selected and the current supply time Tis set to T1 (e.g., 10 ms).

If CASE=5 (sixth current supply), the routine proceeds to step 212,where V/W-phase current supply is selected and the current supply time Tis set to T2 (e.g., 100 ms).

With the above steps, in the case where the initial drive should beperformed with the P-position, current supply is effected all the phasesaround in order of W-phase→U and W-phases→U-phase→U andV-phases→V-phase→V and W-phases, whereby the rotor 32 is driven in thenormal rotation direction (i.e., the rotation direction from the Pposition to the non-P-position). In this case, the one-phase currentsupply time T1 is set shorter than the two-phase current supply time T2.

On the other hand, if the non-P-position initial drive routine of FIG.11 is activated at step 108, it is determined at steps 221-226 which of“0” to “5”, the count CASE of the current-supply counter is equal to. Acurrent supply phase and a current supply time T are set in thefollowing manner in accordance with a result of the determination of thecount CASE.

If CASE=0 (first current supply), the routine proceeds to step 227,where V-phase current supply is selected and the current supply time Tis set to T1 (e.g., 10 ms).

If CASE=1 (second current supply), the routine proceeds to step 228,where U/V-phase current supply is selected and the current supply time Tis set to T2 (e.g., 100 ms).

If CASE=2 (third current supply), the routine proceeds to step 229,where U-phase current supply is selected and the current supply time Tis set to T1 (e.g., 10 ms).

If CASE=3 (fourth current supply), the routine proceeds to step 230,where U/W-phase current supply is selected and the current supply time Tis set to T2 (e.g., 100 ms).

If CASE=4 (fifth current supply), the routine proceeds to step 231,where W-phase current supply is selected and the current supply time Tis set to T1 (e.g., 10 ms).

If CASE=5 (sixth current supply), the routine proceeds to step 232,where V/W-phase current supply is selected and the current supply time Tis set to T2 (e.g., 100 ms).

With the above steps, in the case where the initial drive should beperformed with the non-P-position, current supply is effected all thephases around in order of V-phase→U and V-phases→U-phase→U andW-phases→W-phase→V and W-phases, whereby the rotor 32 is driven in thereverse rotation direction (i.e., the rotation direction from the non-Pposition to the P-position). Also in this case, the one-phase currentsupply time T1 is set shorter than the two-phase current supply time T2.

After the P-position initial drive routine of FIG. 10 or thenon-P-position initial drive routine of FIG. 11 is executed in the abovemanner, the routine proceeds to step 109 shown in FIG. 8, where it isdetermined whether a position switching manipulation (i.e., amanipulation on the P-position switch 43 or the non-P-position switch44) has been performed during the initial drive. If a position switchingmanipulation has been performed during the initial drive, the routineproceeds to step 110, where a position switching manipulation flag Xchgis set to “on.” If no position switching manipulation has beenperformed, the routine proceeds to step 111, where the positionswitching manipulation flag Xchg is set to “off.”

Then, the routine proceeds to step 112 shown in FIG. 9, where the countCT of a counter that counts the current supply time of the currentsupply phase is incremented. At the next step 113, it is determinedwhether the current supply time CT of the current supply phase hasexceeded the current supply time T that was set in the routine of FIG.10 or 11. If the current supply time CT has not exceeded the currentsupply time T, this routine is finished without executing the remainingsteps. In this manner, current supply is continued for the currentsupply phase until the current supply time CT of the current supplyphase exceeds the current supply time T (=T1 or T2) that was set in theroutine of FIG. 10 or 11.

Then, at an instant when the current supply time CT of the currentsupply phase has exceeded the current supply time T (=T1 or T2) that wasset in the routine of FIG. 10 or 11, the routine proceeds to step 114,where the count CASE of the current-supply counter is incremented by oneand the current supply phase is switched to the next phase. At the nextstep 115, the count CT of the current supply time counter is reset. Atthe next step 116, it is determined whether the count CASE of thecurrent-supply counter has reached “6” meaning that the initial driveshould be finished. If the count CASE has reached “6,” the routineproceeds to step 118, where an initial drive end flag Xend is set to“on” meaning that the initial drive should be finished.

If the count CASE has not reached “6” yet, that is, if the initial driveshould be continued, the routine proceeds to step 117, where it isdetermined whether a condition for terminating the initial drive halfwayis satisfied. Whether the condition for terminating the initial drivehalfway is satisfied is determined according to the following threecriteria:

(1) The range determination flag Xnp is “0” (P-position).

(2) The count CASE of the current-supply is equal to 2 or 4, that is,two-phase current supply has finished.

(3) The position switching manipulation flag Xchg is “on,” that is, aposition switching manipulation has been performed during the initialdrive.

If at least one of the criteria (1) to (3) is not met, the condition forterminating the initial drive halfway is not satisfied and hence theinitial drive is continued. In contrast, if all the three criteria (1)to (3) are met, the condition for terminating the initial drive halfwayis satisfied and the routine proceeds to step 118, where the initialdrive end flag Xend is set to “on” meaning that the initial drive shouldbe finished.

Then, the routine proceeds to step 119, where it is determined whetherthe range determination flag Xnp is “1” (i.e., whether the initial drivehas been performed with the non-P-position). If the range determinationflag Xnp is “1,” the routine proceeds to step 120, where an encodercount Ncnt at the end of the initial drive is stored as an initialpositional deviation learned value Gcnt. At the next step 121, theencoder count Ncnt is corrected to a value having the non-P position asa reference according to the following equation:Ncnt=Ncnt+288

In this embodiment, if the encoder count Ncnt is increased starting fromthe P-position holding position that is the zero-count position, theencoder count Ncnt becomes 288, for example, when the rotor 32 hasrotated to the non-P-position holding position. Therefore, if theinitial drive is performed with the non-P-position, an encoder countNcnt at the end of the initial drive is corrected to a value having thenon-P position as a reference by adding 288 to it.

On the other hand, if it is determined at step 119 that the rangedetermination flag Xnp is “0” (the initial drive has been performed withthe P-position), the routine proceeds to step 122, where an initialpositional deviation learned value Gcnt is calculated according to thefollowing equation by using the encoder count Ncnt at the end of theinitial drive:Gcnt=Ncnt+2×(6−CASE)

If the initial drive has finished without being terminated halfway, thecount CASE should be equal to 6 because of the repeated execution ofstep 114 and an encoder count at the end of the initial drive isemployed as an initial positional deviation learned value Gcnt as it is.However, where the initial drive is performed with the P-position, if aposition switching manipulation is performed during the initial drive, atransition is made to an ordinary drive after completion of the nexttwo-phase current supply (after completion of the current two-phasecurrent supply if a position switching request occurs during the currenttwo-phase current supply). Therefore, an encoder count Ncnt that wouldbe obtained if the initial drive were finished without being terminatedhalfway is inferred and an inferred value is employed as an initialpositional deviation learned value Gcnt. The term “2×(6−CASE)” is acount increase (herein after referred to as “Ncnt correction value”)corresponding to a rotation angle of current supply attempts that werenot made.

FIG. 14 is a time chart illustrating a relationship among the count CASEof the current-supply counter, the Ncnt correction value, the currentsupply phase, the A-phase signal, the B-phase signal, and the encodercount Ncnt in an initial drive. For example, when an initial drive isfinished at the end of U/W-phase current supply (the count CASE of thecurrent-supply counter changes from 1 to 2), the Ncnt correction valuebecomes 2×(6−CASE)=2×(6-2)=8. When an initial drive is finished at theend of U/V-phase current supply (the count CASE of the current-supplycounter changes from 3 to 4), the Ncnt correction value becomes2×(6−CASE)=2×(6−4)=4.

[Encoder Counter]

Next, the details of an encoder counter routine shown in FIG. 15 will bedescribed. This routine is activated by A/B-phase interruptionprocessing in synchronism with both of rising edges and trailing edgesof an A-phase signal and a B-phase signal and counts both of risingedges and trailing edges in the following manner. Upon activation ofthis routine, at step 301, a value A(i) of the A-phase signal and avalue B(i) of the B-phase signal are read. At the next step 302, a countincrement ΔN corresponding to the current values A(i) and B(i) andpreceding values A(i−1) and B(i−1) of the A-phase signal and the B-phasesignal is calculated by searching a count increment ΔN calculation mapshown in FIG. 16.

The reason for using the current values A(i) and B(i) and the precedingvalues A(i−1) and B(i−1) of the A-phase signal and the B-phase signal isto determine a rotation direction of the rotor 32 on the basis of orderof occurrence of the A-phase signal and the B-phase signal. As shown inFIG. 17, in the case of normal rotation (i.e., the rotation directionfrom the P-position to the non-P-position), the encoder count Ncnt isincreased by giving the plus sign to the count increment ΔN. In the caseof reverse rotation (i.e., the rotation direction from thenon-P-position to the P-position), the encoder count Ncnt is decreasedby giving the minus sign to the count increment ΔN.

After the calculation of the count increment ΔN, the routine proceeds tostep 303, where a new encoder count Ncnt is calculated by adding thecount increment ΔN as calculated at step 302 to the preceding encodercount Ncnt. At the next step 304, for the next counting processing, thecurrent values A(i) and B(i) of the A-phase signal and the B-phasesignal are stored as values A(i−1) and B(i−1), respectively.

[Control Mode Setting]

A control mode setting routine shown in FIGS. 20-22, which is executedin a prescribed cycle (e.g., every 1 ms) after the end of an initialdrive, specifies a control mode in the following manner by setting acontrol mode determination value “mode” to one of 0, 1, 3, 4, and 5.

mode=0: current supply off (standby)

mode=1: ordinary drive (feedback control start position stopping andholding process and feedback control)

mode=3: target position stopping and holding process

mode=4: reversing position stopping and holding process

mode=5: open-loop control

Upon activation of the control mode setting routine, at step 401, it isdetermined whether a system failure flag Xfailoff is set to “on” meaningthat the position switching controller 42 is in failure. If the flagXfailoff is “on,” the routine proceeds to step 402, where a process forkeeping the SR motor 12 in a current supply-off state, whereupon thefollowing settings are made: a rotation direction instruction value D=0(stop), a current supply flag Xon=off (current supply off), a feedbackpermission flag Xfb=off (a feedback control prohibited), and the controlmode determination value “mode”=0 (current supply off).

On the other hand, if the system failure flag Xfailoff is “off” (nofailure), the routine goes from step 401 to step 403, where it isdetermined whether the open-loop control execution flag Xopen is “off”and the recovery process execution flag Xrcv is “off.” If one or both ofthe open-loop control execution flag Xopen and the recovery processexecution flag Xrcv are “on,” the routine proceeds to step 404, wherethe following settings are made to execute an open-loop control: therotation direction instruction value D=0 (stop), the control modedetermination value “mode”=5 (open-loop control), and the feedbackpermission flag Xfb=off (a feedback control prohibited).

If both of the open-loop control execution flag Xopen and the recoveryprocess execution flag Xrcv are “off,” the routine proceeds to step 405,where it is determined whether the current supply flag Xon is “on”(current supply on). If the current supply flag Xon is “off” (currentsupply off), the routine proceeds to step 406, where the differencebetween a target count Acnt and an encoder count Ncnt (i.e., thedifference between a target position and a position of the rotor 32) iscalculated and it is determined on the basis of the difference Acnt−Ncntwhich of normal rotation (i.e., rotation from the P-position to thenon-P-position), reverse rotation (i.e., rotation from thenon-P-position to the P-position), and a stop the current rotation modeis. At this time, a value as corrected by using an initial positionaldeviation learned value Gcnt that was learned in the initial driveroutine of FIGS. 8 and 9 is used as the encoder count Ncnt:Ncnt=Ncnt−GcntThe target count Acnt is set by a target count setting routine(described later) shown in FIGS. 45 and 46.

If the difference Acnt−Ncnt between the target count Acnt and theencoder count Ncnt is greater than or equal to +Kth (e.g., +10°), it isdetermined that the rotor 32 needs to be rotated in the normal direction(i.e., the rotation direction from the P-position to thenon-P-position). The routine proceeds to step 407, where the followingsettings are made: the rotation direction instruction value D=1 (normalrotation), the current supply flag Xon=on (current supply on), and thecontrol mode determination value “mode”=1 (feedback control startposition stopping and holding process and feedback control).

If the difference Acnt−Ncnt between the target count Acnt and theencoder count Ncnt is smaller than or equal to −Kth (e.g., −10°), it isdetermined that the rotor 32 needs to be rotated in the reversedirection (i.e., the rotation direction from the non-P-position to theP-position). The routine proceeds to step 409, where the followingsettings are made: the rotation direction instruction value D=−1(reverse rotation), the current supply flag Xon=on (current supply on),and the control mode determination value “mode”=1 (feedback controlstart position stopping and holding process and feedback control).

If the difference Acnt−Ncnt between the target count Acnt and theencoder count Ncnt is in a range of −Kth to +Kth (e.g., −10° to +10°),it is determined that the rotor 32 can be kept at the target position byspring force of the detent spring 23 (i.e., it is not necessary toenergize the SR motor 12). The routine proceeds to step 408, where thefollowing settings are made to keep the SR motor 12 in a currentsupply-off state: the rotation direction instruction value D=0 (stop),the current supply flag Xon=off (current supply off), and the controlmode determination value “mode”=0 (current supply off).

On the other hand, if it is determined at step 405 that the currentsupply flag Xon is “on” (current supply on), steps 410-415 shown in FIG.21 are executed, whereby it is determined by whether the instructedshift position (target position) has been reversed and, if theinstructed shift position has been reversed, the rotation directioninstruction value D is reversed.

More specifically, first, it is determined at step 410 whether therotation direction instruction value D is equal to “1” (normalrotation). If the rotation direction instruction value D is equal to “1”(normal rotation), the routine proceeds to step 411, where whether it isnecessary to change the rotation direction of the rotor 32 from thenormal direction to the reverse direction is determined on the basis ofwhether the difference Acnt−Ncnt between the target count Acnt and theencoder count Ncnt has a negative value. If it is necessary to changethe rotation direction of the rotor 32 that way, the routine proceeds tostep 412, where the rotation direction instruction value D is set to“−1” (reverse rotation).

In contrast, if it is determined at step 410 that the rotation directioninstruction value D is not equal to “1” (normal rotation), that is, ifit is determined that the value D is equal to “0” or “−1,” the routineproceeds to step 413, where it is determined whether the rotationdirection instruction value D is equal to “−1” (reverse rotation). Ifthe value D is equal to “−1” (reverse rotation), the routine proceeds tostep 414, where whether it is necessary to change the rotation directionof the rotor 32 from the reverse direction to the normal direction isdetermined on the basis of whether the difference Acnt−Ncnt between thetarget count Acnt and the encoder count Ncnt has a positive value. If itis necessary to change the rotation direction of the rotor 32 that way,the routine proceeds to step 415, where the rotation directioninstruction value D is set to “1” (normal rotation).

If the rotation direction instruction value D has been reversed in theabove manner, the routine proceeds to step 416, where the followingsettings are made to reverse the rotation direction of the rotor 32: thecontrol mode determination value “mode”=4 (reversing position stoppingand holding process) and the feedback permission flag Xfb=off (feedbackcontrol prohibited). Then, the routine proceeds to step 417. On theother hand, if the rotation direction instruction value D has not beenreversed, the routine proceeds to step 417 skipping step 416.

At step 417, it is determined whether the control mode determinationvalue “mode” is equal to “4” (reversing position stopping and holdingprocess). If the determination result is “yes,” the routine proceeds tostep 418, where the current supply flag Xon is set to “on” (currentsupply on), whereupon a reversing position stopping and holding processis executed.

On the other hand, if the determination result at step 417 is “no”(i.e., a reversing position stopping and holding process is not to beexecuted), steps 419-421 shown in FIG. 22 are executed, whereby it isdetermined whether the feedback control should be finished.Specifically, it is determined at step 419 whether the rotationdirection instruction value D is greater than or equal to “0” (normalrotation or a stop). If the value D≧0, the routine proceeds to step 420,where whether the feedback control should be finished is determined onthe basis of whether the difference Acnt−Ncnt between the target countAcnt and the encoder count Ncnt is smaller than or equal to +Kref (e.g.,+0.5°). If the rotation direction instruction value D is equal to “−1”(reverse rotation), the routine proceeds to step 421, where whether thefeedback control should be finished is determined on the basis ofwhether the difference Acnt−Ncnt between the target count Acnt and theencoder count Ncnt is greater than or equal to −Kref (e.g., −0.5°).

That is, by setting the feedback control end determination value Kref tothe phase lead (e.g., 2 to 4 counts) of the current supply phase, thefeedback control is finished at an instant that precedes, by a timecorresponding to the phase lead of the current supply phase, an instantthat is determined by the target count Acnt. With this measure, the lastcurrent supply phase of the feedback control comes to coincide with acurrent supply phase with which the rotor 32 is to be stopped and keptat the target position (target count Acnt).

If the determination result at step 420 or 421 is “no” (i.e., thefeedback control should not be finished), the routine proceeds to step422, where the count CThold of a stopping and holding time counter forcounting the time of a target position stopping and holding process isreset.

On the other hand, if the determination result at step 420 or 421 is“yes” (i.e., the feedback control should be finished), the routineproceeds to step 423, where the feedback permission flag Xfb is set to“off” (feedback control prohibited), whereupon the feedback control isfinished and a transition is made to a target position stopping andholding process. At the next step 424, the count CThold of the stoppingand holding time counter is incremented to count the time of the targetposition stopping and holding process.

Then, at step 425, it is determined whether the time CThold of thetarget position stopping and holding process has reached a prescribedtime (e.g., 50 ms). If the time CThold has not reached the prescribedtime (e.g., 50 ms) yet, the routine proceeds to step 426, where thefollowing settings are maintained to continue the target positionstopping and holding process: the rotation direction instruction valueD=0 (stop), the current supply flag Xon=on (current supply on), and thecontrol mode determination value “mode”=3 (target position stopping andholding process).

When the time CThold of the target position stopping and holding processhas reached the prescribed time (e.g., 50 ms), the routine proceeds tostep 427, where the following settings are made to turn off the currentsupply of the SR motor 12: the rotation direction instruction value D=0(stop), the current supply flag Xon=off (current supply off), and thecontrol mode determination value “mode”=0 (current supply off).

[Time-Synchronous Motor Control]

A time-synchronous motor control routine shown in FIG. 23 is activatedin a prescribed cycle (e.g., every 1 ms) after the end of an initialdrive. A feedback control start position stopping and holding process, atarget position stopping and holding processing, or a reversing positionstopping and holding process is executed in this routine.

Upon activation of this routine, at step 501, it is determined whetherthe feedback permission flag Xfb is “off” (feedback control prohibited).If the feedback permission flag Xfb is “on” (feedback controlpermitted), the routine is finished without executing the remainingsteps. In this case, current supply phase setting and current supplyprocessing are performed by a feedback control routine (described later)shown in FIG. 30.

On the other hand, if it is determined at step 501 that the feedbackpermission flag Xfb is “off” (feedback control prohibited), it isdetermined at steps 502-504 whether the control mode determination value“mode” is equal to 1, 3, or 4. If the control mode determination value“mode” is equal to “1” (feedback control start position stopping andholding process and feedback control), the routine goes from step 502 tostep 505, where a mode-1 routine (described later) shown in FIG. 24 isexecuted, whereby a current supply phase determination value Mptnindicating a current supply phase with which to execute a feedbackcontrol start position stopping and holding process is calculated.

If the control mode determination value “mode” is equal to “3” (targetposition stopping and holding process), the routine goes from step 503to step 506, where a mode-3 routine (described later) shown in FIG. 25is executed, whereby a current supply phase determination value Mptnindicating a current supply phase with which to execute a targetposition stopping and holding process is calculated.

If the control mode determination value “mode” is equal to “4”(reversing position stopping and holding process), the routine goes fromstep 504 to step 507, where a mode-4 routine (described later) shown inFIG. 26 is executed, whereby a current supply phase determination valueMptn indicating a current supply phase with which to execute a currentsupply position stopping and holding process is calculated.

As described above, if the control mode determination value “mode” isequal to 1, 3, or 4, a current supply phase determination value Mptn iscalculated and the routine proceeds to step 508, where a current supplyprocessing routine shown in FIG. 27 is executed, that is, a feedbackcontrol start position stopping and holding process, a target positionstopping and holding processing, or a reversing position stopping andholding process is executed.

On the other hand, if the determination results at all of steps 502-504are “no,” that is, if the control mode determination value “mode” isequal to 0 or 5, the routine directly proceeds to step 508, where thecurrent supply processing routine shown in FIG. 27 is executed, that is,the current supply is turned off or an open-loop control is performed.

[Mode-1]

The mode-1 routine shown in FIG. 24 is a subroutine that is activated atstep 505 of the time-synchronous motor control routine of FIG. 23. Inthe mode-1 routine, a current supply phase determination value Mptn(current supply phase) for a feedback control start position stoppingand holding process is set in the following manner.

Upon activation of this routine, at step 511, the count CT1 of a currentsupply time counter that counts the time of a feedback control startposition stopping and holding process is incremented. At the next step512, it is determined whether the time CT1 of the feedback control startposition stopping and holding process has exceeded a prescribed time(e.g., 10 ms).

If the time CT1 of the feedback control start position stopping andholding process has not exceeded the prescribed time (e.g., 10 ms), theroutine proceeds to step 513, where it is determined whether a stoppingand holding current supply phase storage flag Xhold is “off” (notstored), that is, whether the present instant is immediately before astart of the feedback control start position stopping and holdingprocess. If the stopping and holding current supply phase storage flagXhold is “off,” the routine proceeds to step 514, where a currentposition count Ncnt−Gcnt is set as a current supply phase determinationvalue Mptn for the feedback control start position stopping and holdingprocess:Mptn=Ncnt−Gcnt

The current position count Ncnt−Gcnt is an encoder count Ncnt ascorrected by using an initial positional deviation learned value Gcnt,and represents a current position of the rotor 32 correctly.

Then, the routine proceeds to step 515, where the current supply phasedetermination value Mptn is divided by 12 and a remainder Mptn %12 isobtained. The number “12” is an increase or decrease of the encodercount Ncnt (current supply phase determination value Mptn) that occurswhen current supply is effected for all the phases around. A currentsupply phase is determined according to a conversion table shown in FIG.28 on the basis of the value Mptn %12.

At step 516, whether one-phase current supply (U-phase, V-phase, orW-phase) is going to be performed is determined on the basis of whetherthe value Mptn %12 is equal to 2, 3, 6, 7, 10, or 11. If one-phasecurrent supply is going to be performed, the routine proceeds to step517, where the current supply phase determination value Mptn isincremented by 2 that corresponds to one step so that two-phase currentsupply (U and V-phases, V and W-phases, or U and W-phases) will beperformed. Executing the feedback control start position stopping andholding process with two-phase current supply which produces highertorque than one-phase current supply does prevents the rotor 32 fromvibrating in the vicinity of a feedback control start position and hencecan reliably stop and keep the rotor 32 at the feedback control startposition.

The stopping and holding current supply phase storage flag Xhold is setto “on” (stored) at the next step 518, Then this routine is finished.When this routine is activated later, the determination result at step513 should become “no” and hence steps 514-518 are not executed. Thatis, the process of setting a current supply phase determination valueMptn (current supply phase) for the feedback control start positionstopping and holding process is set is performed only once immediatelybefore a start of the feedback control start position stopping andholding process.

Then, when the time CT1 of the feedback control start position stoppingand holding process has exceeded the prescribed time (e.g., 10 ms), thedetermination result at step 512 becomes “yes,” whereupon the feedbackcontrol start position stopping and holding process is finished and atransition is made to a feedback control. First, at step 519, a firstcurrent supply phase determination value Mptn with which to start thefeedback control is set by adding or subtracting, in accordance with arotation direction, a count (e.g., 4 or 3) corresponding to A-phase leadof the current supply phase to or from the current supply phasedetermination value Mptn for the feedback control start positionstopping and holding process, whereupon driving for rotating the rotor32 is started. Then, the routine proceeds to step 520, where thefeedback permission flag Xfb is set to “on” (feedback controlpermitted).

FIG. 32 is a time chart illustrating A-phase for which current supply iseffected first in the case where rotation is started from a statecorresponding to the U and W-phases. In this case, to start normalrotation (i.e., rotation from the P-position to the non-P-position), acurrent supply phase determination value Mptn is calculated according tothe following equation by using an encoder count Ncnt, an initialpositional deviation learned value Gcnt, and a normal rotation directionphase lead K1:Mptn=Ncnt−Gcnt+K1

If the normal rotation direction phase lead K1 is equal to 4, forexample, a current supply phase determination value Mptn is calculatedaccording to the following equation:Mptn=Ncnt−Gcnt+4To start normal rotation from a state corresponding to the U andW-phases, mod(Ncnt−Gcnt) is 4 and hence Mptn %12 is equal to 8 (=4+4).The U and V-phases are set as first current supply phases.

On the other hand, to start reverse rotation (i.e., rotation from thenon-P-position to the P-position) from a state corresponding to the Uand W-phases, if a reverse rotation direction phase lead K2 is equal to3, for example, a current supply phase determination value Mptn iscalculated according to the following equation:

Mptn = Ncnt − Gcnt − K 2     = Ncnt − Gcnt − 3To start reverse rotation from a state corresponding to the U andW-phases, Mptn %12 is equal to 1 (=4−3). The V and W-phases are set asfirst current supply phases.

In this manner, by setting the normal rotation direction phase lead K1and the reverse rotation direction phase lead K2 to 4 and 3,respectively, current supply phase switching patterns for the normalrotation direction and the reverse rotation direction can be madesymmetrical. In either of the normal rotation direction and the reverserotation direction, rotation can be started by effecting current supplyfirst for A-phase of a position that is two steps deviated from thecurrent position of the rotor 32.

[Mode-3]

The mode-3 routine shown in FIG. 25 is a subroutine that is activated atstep 506 of the time-synchronous motor control routine of FIG. 23. Inthe mode-3 routine, a current supply phase determination value Mptn(current supply phase) for a target position stopping and holdingprocess is set in the following manner.

Upon activation of this routine, at step 531, whether the current supplyphase at the end of the feedback control is one phase (U-phase, V-phase,or W-phase) is determined on the basis of whether the value Mptn %12 isequal to 2, 3, 6, 7, 10, or 11. If it is one-phase, steps 532-534 areexecuted, whereby the current supply phase determination value Mptn isincremented or decremented by 2 in accordance with the rotationdirection of the feedback control performed so far and the currentsupply phase is thereby changed to two phases next to the current onephase.

In doing so, at step 532 the rotation direction is determined in thefollowing manner. The rotation direction instruction value D is set to“0” (stop) at step 426 in FIG. 22 immediately before this routine isactivated (i.e., when the feedback control has finished). Therefore, therotation direction cannot be determined by checking the rotationdirection instruction value D. In view of this, in this embodiment, withattention paid to the fact that the current supply phase determinationvalue Mptn at the end of the feedback control and the position countNcnt−Gcnt are different from each other by the phase lead K1 or K2 ofthe current supply phase, the rotation direction is determined in thefollowing manner in accordance with a magnitude relationship between thecurrent supply phase determination value Mptn at the end of the feedbackcontrol and the position count Ncnt−Gcnt.

If Mptn>(Ncnt−Gcnt), the rotation direction is determined to be normalrotation (i.e., the rotation direction from the P-position to thenon-P-position). The routine proceeds to step 533, where the currentsupply phase determination value Mptn is incremented by 2 to as toeffect two-phase current supply.

On the other hand, if Mptn<(Ncnt−Gcnt), the rotation direction isdetermined to be reverse rotation (i.e., the rotation direction from thenon-P-position to the P-position). The routine proceeds to step 534,where the current supply phase determination value Mptn is decrementedby 2 to as to effect two-phase current supply. If Mptn=(Ncnt−Gcnt), itis determined that the rotor 32 is stopped and hence the current supplyphase is not changed.

As described above, like the feedback control start position stoppingand holding process, the target position stopping and holding process isexecuted with two-phase current supply which produces higher torque thanone-phase current supply does. This makes it possible to prevent therotor 32 from vibrating in the vicinity of a target position and henceto reliably stop and keep the rotor 32 at the target position.

[Mode-4]

The mode-4 routine shown in FIG. 26, which is a subroutine that isactivated at step 507 of the time-synchronous motor control routine ofFIG. 23, sets a current supply phase determination value Mptn for areversing position stopping and holding process in the following manner.

Upon activation of this routine, at step 541, the count CT4 of a currentsupply time counter that counts the time of a reversing positionstopping and holding process. At the next step 542, it is determinedwhether the time CT4 of the reversing position stopping and holdingprocess has exceeded a prescribed time (e.g., 50 ms).

If the time CT4 of the reversing position stopping and holding processhas not exceeded the prescribed time (e.g., 50 ms), the routine proceedsto step 543, where whether the current supply phase is one phase(U-phase, V-phase, or W-phase) is determined on the basis of whether thevalue Mptn %12 is equal to 2, 3, 6, 7, 10, or 11. If the current supplyphase is one phase, steps 544-546 are executed, whereby the currentsupply phase determination value Mptn is incremented or decremented by 2in accordance with the rotation direction of the feedback controlperformed so far and the current supply phase is thereby changed to twophases next to the current one phase. Steps 543-546 are the same assteps 531-534 of the above mode-3 routine of FIG. 25.

As described above, like the feedback control start position stoppingand holding process and the target position stopping and holdingprocess, the reversing position stopping and holding process is executedwith two-phase current supply which produces higher torque thanone-phase current supply does. This makes it possible to prevent therotor 32 from vibrating in the vicinity of a reversing position andhence to reliably stop the rotor 32 at the reversing position and keepit there.

Then, when the time CT4 of the reversing position stopping and holdingprocess has exceeded the prescribed time (e.g., 50 ms), thedetermination result at step 542 becomes “yes,” whereupon the reversingposition stopping and holding process is finished and the feedbackcontrol is restarted. First, at step 547, a first current supply phasedetermination value Mptn for the new feedback control is set by addingor subtracting, in accordance with a rotation direction, a count (e.g.,4 or 3) corresponding to A-phase lead of the current supply phase to orfrom the current supply phase determination value Mptn for the reversingposition stopping and holding process, whereupon driving for rotatingthe rotor 32 is started. Then, the routine proceeds to step 548, wherethe following settings are made: the feedback permission flag Xfb=on(feedback control permitted), the current supply time count CT4=0, andthe control mode determination value “mode”=1 (ordinary drive). Thenthis routine is finished.

[Current Supply Process]

The current supply processing routine shown in FIG. 27 is a subroutinethat is activated at step 508 of the time-synchronous motor controlroutine of FIG. 23. This routine is also activated at step 603 of thefeedback control routine (described later) shown in FIG. 30.

Upon activation of the current supply processing routine of FIG. 27, atstep 551, it is determined whether the control mode determination value“mode” is equal to “0” (current supply off). If the control modedetermination value “mode” is equal to “0” (current supply off), theroutine proceeds to step 552, where all the phases are rendered in acurrent supply-off state to establish a standby state.

On the other hand, if the determination result at step 551 is “no,” theroutine proceeds to step 553, where it is determined whether the controlmode determination value “mode” is equal to “5” (open-loop control). Ifthe control mode determination value “mode” is equal to “5” (open-loopcontrol), the routine proceeds to step 554, where an open-loop controlis performed. In the open-loop control, the rotor 32 is rotated to atarget position by setting current supply phases by time-synchronousprocessing having a cycle of 1 ms, for example, when the encoder 46 hasfailed or an operation abnormality has occurred in the SR motor 12.

If the determination results at both of steps 551 and 553 are “no,” thatis, if the control mode determination value “mode” is equal to 1(feedback control start position stopping and holding process andfeedback control), 3 (target position stopping and holding process), or4 (reversing position stopping and holding process), the routineproceeds to step 555, where a current supply phase is set in accordancewith a value Mpt %12 using the conversion table of FIG. 28.

Then, the routine proceeds to step 556, where it is determined whether abutting control is being performed (i.e., whether a P-position-sidebutting control flag Xexp or a non-P-position-side butting control flagXexnp is “on”). If no butting control is being performed, the routineproceeds to step 557, where the current supply duty ratio of the SRmotor 12 is set to 100%. If a butting control is being performed, theroutine proceeds to step 558, where the current supply duty ratio of theSR motor 12 is set to 10% to 30%, for example, in accordance with abattery voltage as a power source voltage for the SR motor 12 by using atable of FIG. 29. As a result, during a butting control, the torque ofthe SR motor 12 is made much lower than in an ordinary drive.

In the exemplary table of FIG. 29, in view of the characteristic thatthe torque of the SR motor 12 increases as the power source voltage forthe SR motor 12 (i.e., the battery voltage) increases, the currentsupply duty ratio is set smaller as the power source voltage for the SRmotor 12 (i.e., the battery voltage) increases. This makes the torque ofthe SR motor 12 approximately constant irrespective of whether the powersource voltage (i.e., the battery voltage) is high or low, and therebyallows the SR motor 12 to always generate minimum torque that isnecessary for a butting control.

After the current supply duty ratio has been set in the above manner,the routine proceeds to step 559, where the ECU 41 outputs controlsignals to the motor drivers 37 and 38, whereby the windings of thecurrent supply phase that was set at step 555 are energized with thecurrent supply duty ratio that was set at step 557 or 558 and the SRmotor 12 is thereby driven.

[Feedback Control]

Next, the details of the feedback control routine shown in FIG. 30 willbe described. This routine is executed by A/B-phase interruptionprocessing. In this routine, in a state that the feedback controlexecution conditions are satisfied after the end of an initial drive,the rotor 32 is rotated until its rotation position (Ncnt−Gcnt) reachesa position that is 0.5°, for example, short of a target position (targetcount Acnt) by switching the current supply phase on the basis of theencoder count Ncnt and an initial positional deviation learned valueGcnt.

Upon activation of the feedback control routine of FIG. 30, at step 601,it is determined whether the feedback permission flag Xfb is “on” (i.e.,whether the feedback control execution conditions are satisfied). If thefeedback permission flag Xfb is “off” (i.e., the feedback controlexecution conditions are not satisfied), this routine is finishedwithout executing the remaining steps.

On the other hand, if the feedback permission flag Xfb is “on,” theroutine proceeds to step 602, where a current supply phase settingroutine (described later) shown in FIG. 31 is executed, whereby acurrent supply phase is set on the basis of a current encoder count Ncntand an initial positional deviation learned value Gcnt. At the next step603, the current supply processing routine of FIG. 27 is executed.

[Current Supply Phase Setting]

The current supply phase setting routine of FIG. 31 is a subroutine thatis activated at step 602 of the feedback control routine of FIG. 30.Upon activation of this routine, at step 611, it is determined whetherthe rotation direction instruction value D indicating a rotationdirection toward a target position is equal to “1” that means the normalrotation direction (i.e., the rotation direction from the P-position tothe non-P-position) If it is determined that the rotation directioninstruction value D is equal to “1” (normal rotation), the routineproceeds to step 612, where it is determined whether the rotationdirection of the rotor 32 has reversed (i.e., whether the encoder countNcnt has decreased) contrary to the rotation direction instruction. Ifthe rotation direction has not reversed, the routine proceeds to step613, where the current supply phase determination value Mptn is updatedaccording to the following equation by using a current encoder countNcnt, an initial positional deviation learned value Gcnt, a normalrotation direction phase lead K1, and a speed phase lead correctionamount Ks:Mptn=Ncnt−Gcnt+K1+Ks

The normal rotation direction phase lead K1 is A-phase lead of thecurrent supply phase that is necessary to rotate the rotor 32 in thenormal direction (i.e., A-phase lead of the current supply phase withrespect to the current rotation phase of the rotor 32), and is set to“4,” for example.

The speed phase lead correction amount Ks is A-phase lead correctionamount that is set in accordance with a rotation speed of the rotor 32,and is set by a speed phase lead correction amount setting routine(described later) shown in FIG. 34. For example, the speed phase leadcorrection amount Ks is set to “0” in a low-speed range and is increasedto “1” or “2,” for example, as the rotation speed increases. As aresult, the current supply phase determination value Mptn is correctedso as to indicate a current supply phase that is suitable for a rotationspeed of the rotor 32.

On the other hand, if it is determined at step 612 that the rotationdirection of the rotor 32 has reversed contrary to the rotationdirection instruction, the current supply phase determination value Mptnis not updated to prevent progress of the reversed rotation. In thiscase, current supply is effected for the phase for which current supplywas effected immediately before the reversing (i.e., the precedingcurrent supply phase), whereby braking torque is generated in such adirection as to suppress the reversed rotation.

If it is determined at step 611 that the rotation direction instructionvalue D is equal to “−1” (the reverse rotation direction, i.e., therotation direction from the non-P-position to the P-position), theroutine proceeds to step 614, where it is determined whether therotation direction of the rotor 32 has reversed (i.e., whether theencoder count Ncnt has increased) contrary to the rotation directioninstruction. If the rotation direction has not reversed, the routineproceeds to step 615, where the current supply phase determination valueMptn is updated according to the following equation by using a currentencoder count Ncnt, an initial positional deviation learned value Gcnt,a reverse rotation direction phase lead K2, and a speed phase leadcorrection amount Ks:Mptn=Ncnt−Gcnt−K2−Ks

The reverse rotation direction phase lead K2 is A-phase lead of thecurrent supply phase that is necessary to rotate the rotor 32 in thereverse direction (i.e., A-phase lead of the current supply phase withrespect to the current rotation phase of the rotor 32), and is set to“3,” for example. As in the case of the normal rotation, the speed phaselead correction amount Ks is set by the speed phase lead correctionamount setting routine (described later) shown in FIG. 34.

On the other hand, if it is determined at step 614 that the rotationdirection of the rotor 32 has reversed contrary to the rotationdirection instruction, the current supply phase determination value Mptnis not updated to prevent progress of the reversed rotation (i.e.,normal rotation). In this case, current supply is effected for the phasefor which current supply was effected immediately before the reversing(i.e., the preceding current supply phase), whereby braking torque isgenerated in such a direction as to suppress the reversed rotation(i.e., normal rotation).

After the current supply phase determination value Mptn has beendetermined in the above manner, the current supply processing routine ofFIG. 27 is executed. While the feedback control is being performed, acurrent supply phase corresponding to a value Mptn %12 is selected atstep 555 by searching the conversion table of FIG. 28 and current supplyis effected for the selected current supply phase.

[Rotor Rotation Speed Calculation]

A rotor rotation speed calculation routine shown in FIG. 33, which isexecuted by A/B-phase interruption processing, calculates a rotationspeed SP of the rotor 32 in the following manner. Upon activation ofthis routine, at step 621, it is determined whether the feedbackpermission flag Xfb is “on” (a feedback control is being performed). Ifthe feedback permission flag Xfb is “off” (feedback prohibited), nocorrection is performed on A-phase lead of the current supply phase inaccordance with a rotation speed SP of the rotor 32 and hence theroutine proceeds to step 624, where stored rotation speeds SP and SPa ofthe rotor 32 are reset. Then this routine is finished.

On the other hand, if the feedback permission flag Xfb is “on” (afeedback control is being performed), a rotation speed SP of the rotor32 is calculated in the following manner. First, at step 622, timeintervals ΔT(n) between rising/trailing edges of an A-phase signal and aB-phase signal of the encoder 46 (i.e., time interval at which theencoder count increases or decreases) are measured and an average ofpast n time intervals ΔT(n) is calculated. And a rotation speedcalculation value SPa is calculated according to the following equation:SPa=60/(ΔTav×Kp) [rpm]

In the above equation, Kp is the number of time intervals ΔT(n) per onerotation of the rotor 32 (i.e., a variation of the encoder count per onerotation of the rotor 32). In the case of the rotor 32 having thestructure of FIG. 5, Kp is equal to 96. And ΔTav×Kp is a time [sec] thatis required for the rotor 32 to make one rotation.

Then, the routine proceeds to step 623, where a rotation speed SP of therotor 32 is calculated through smoothing according to the followingequation by using the rotation speed calculation value SPa:SP(i)=SP(i−1)+{SPa−SP(i−1)}/Rwhere SP(i) is a current rotation speed, SP(i−1) is a preceding rotationspeed, and R is a smoothing coefficient.[Setting of Speed Phase Lead Correction Amount]

The speed phase lead correction amount setting routine shown in FIG. 34,which is activated in a prescribed cycle (e.g., every 1 ms), sets aspeed phase lead correction amount Ks in accordance with a rotationspeed SP of the rotor 32. Upon activation of this routine, at step 631,it is determined whether the feedback permission flag Xfb is “on” (afeedback control is being performed). If the feedback permission flagXfb is off (feedback control prohibited), no correction of A-phase leadof the current supply phase is necessary and hence this routine isfinished without executing the remaining steps.

On the other hand, if the feedback permission flag Xfb is “on” (afeedback control is being performed), steps 632-636 are executed,whereby a speed phase lead correction amount Ks is set in the followingmanner in accordance with the rotation speed SP of the rotor 32 that wascalculated by the rotor rotation speed calculation routine of FIG. 33.

If it is determined at step 632 that the rotation speed SP of the rotor32 is lower than a prescribed value Klow (e.g., 300 rpm), the routineproceeds to step 634, where the speed phase lead correction amount Ks isset to a minimum value Ka (e.g., 0). If it is determined at step 633that the rotation speed SP of the rotor 32 is higher than a prescribedvalue Khigh (e.g., 600 rpm), the routine proceeds to step 636, where thespeed phase lead correction amount Ks is set to a maximum value Kc(e.g., 2). If the rotation speed SP of the rotor 32 is between Klow andKhigh, the routine proceeds to step 635, where the speed phase leadcorrection amount Ks is set to an intermediate value Kb (e.g., 1). Inthis manner, the speed phase lead correction amount KS is set larger asthe rotation speed SP of the rotor 32 increases.

Then, the routine proceeds to step 637, where whether the rotationposition of the rotor 32 has come close to a target position (i.e., hasentered a deceleration range for a stop) is determined on the basis ofwhether the absolute value of the difference between a target count Acntand an encoder count Ncnt, |Acnt−Ncnt|, is smaller than a prescribedvalue.

If |Acnt−Ncnt| is greater than or equal to the prescribed value, theroutine proceeds to step 639, where it is determined whether a buttingcontrol is being performed (i.e., whether the P-position-side buttingcontrol flag Xexp or the non-P-position-side butting control flag Xexnpis “on”). If no butting control is being performed, the speed phase leadcorrection amount Ks that was set in one of steps 634-636 is used as itis. If a butting control is being performed, the routine proceeds tostep 640, where the speed phase lead correction amount Ks is set to asmall value Ke (e.g., 0 or −1) to decrease the rotation speed of therotor 32.

On the other hand, if |Acnt−Ncnt| is smaller than the prescribed value,it is determined that the rotation position of the rotor 32 is in adeceleration range and the routine goes from step 637 to step 638, wherethe speed phase lead correction amount Ks is set to a small value Kd(e.g., 0 or −1).

Although in this routine the speed phase lead correction amount Ks isswitched between three levels in accordance with the rotation speed SPof the rotor 32, it may be switched between two levels or four or morelevels.

[Play Amount Learning]

A play amount learning routine shown in FIG. 36 is executed in aprescribed cycle (e.g., every 8 ms) after the end of an initial drive.Upon activation of this routine, at step 700, it is determined whether aplay amount learning completion flag Xg is “on” (i.e., play amountlearning completed). If the play amount learning completion flag Xg is“on,” this routine is finished without executing the remaining steps. Asa result, play amount learning is performed only once during an ignitionswitch on-period. The play amount learning completion flag Xg is set to“off” by an initialization processing routine (not shown) that isexecuted immediately after the ignition switch is turned on.

On the other hand, if it is determined at step 700 that the play amountlearning completion flag Xg is “off” (i.e., play amount learning has notcompleted), the routine proceeds to step 701, where it is determinedwhether the instructed shift position is the P-position. If it is theP-position, the routine proceeds to step 702, where a P-position-sidebutting control routine of FIGS. 39 and 40 is executed and an encodercount Np at the time of P-position-side butting is stored in the RAM ofthe ECU 41. On the other hand, if the instructed shift position is thenon-P-position, the routine proceeds to step 703, where anon-P-position-side butting control routine of FIGS. 42 and 43 isexecuted and an encoder count Nnp at the time of non-P-position-sidebutting is stored in the RAM of the ECU 41.

Then, the routine proceeds to step 704, where it is determined whetherboth of P-position-side and non-P-position-side butting controls havecompleted (i.e., both of a P-position-side butting control completionflag Xp and a non-P-position-side butting control completion flag Xnpare “on”). If at least one of P-position-side and non-P-position-sidebutting controls has not completed yet, this routine is finished withoutexecuting the remaining steps.

On the other hand, if both of P-position-side and non-P-position-sidebutting controls have completed the routine proceeds to step 705, wherean actual measurement value ΔNact of the movable range of the rotor 32(i.e., the movable range of the detent lever 15) from theP-position-side limit position (i.e., the side wall of the P-positionholding recess 24) to the non-P-position-side limit position (i.e., theside wall of the non-P-position holding recess 25) is calculatedaccording to the following equation:ΔNact=Nnp−Np

In the above equation, Nnp is an encoder count at the time ofnon-P-position-side butting and is set to a value GNnp that is learnedby the non-P-position-side butting control routine (described later) ofFIGS. 42 and 43. And Np is an encoder count at the time ofP-position-side butting and is set to a value GNp that is learned by theP-position-side butting control routine (described later) of FIGS. 39and 40.

After the calculation of the actual measurement value ΔNact of themovable range, the routine proceeds to step 706, where a P-position-sideplay amount ΔGp and a non-P-position-side play amount ΔGnp arecalculated according to the following equation using the actualmeasurement value ΔNact and a design value ΔNd of the movable rangetaking a relationship of FIG. 38 into consideration:ΔGp=ΔGnp=(ΔNact−ΔNd)/2The design value ΔNd of the movable range may be calculated in advanceon the basis of design data or set to a center value of varied movableranges in manufacture of mass-production devices (i.e., an actualmeasurement value of the movable range of a standard device).

As shown in FIG. 38, the difference ΔNact−ΔNd between the actualmeasurement value ΔNact and the design value ΔNd corresponds to the sumΔGp+ΔGnp of the P-position-side play amount ΔGp and thenon-P-position-side play amount ΔGnp. Since the P-position-side playamount ΔGp and the non-P-position-side play amount ΔGnp generallycoincide with each other, each of them can be calculated according tothe above equation.

After the calculation of the play amounts ΔGp and ΔGnp, the routineproceeds to step 707, where the play amount learning completion flag Xgis set to “on” that means completion of play amount learning. Then, thisroutine is finished.

The actual measurement value ΔNact and the play amounts ΔGp and ΔGnpthat have been calculated at steps 705 and 706 are stored, withupdating, in a nonvolatile memory (not shown) such as an SRAM of the ECU41 and the stored values are held even after turning-off of the ignitionswitch. After the ignition switch is turned on next time, a target countAcnt is set by a target count setting routine (described later) shown inFIGS. 45 and 46 by using the actual measurement value ΔNact of themovable range and the play amounts ΔGp and ΔGnp that are stored in thenonvolatile memory of the ECU 41.

[P-Position-Side Butting Control]

The P-position-side butting control routine shown in FIGS. 39 and 40 isa subroutine that is executed at step 702 of the play amount learningroutine of FIG. 36 when the instructed shift position is the P-position.Upon activation of this routine, at step 711, it is determined whetherthe P-position-side butting completion flag Xp is “on” (i.e., aP-position-side butting control has finished). If a P-position-sidebutting control has already finished, this routine is finished withoutexecuting the remaining steps. As a result, a P-position-side buttingcontrol is performed only once during an ignition switch on-period.

On the other hand, if it is determined at step 711 that theP-position-side butting completion flag Xp is “off” (a P-position-sidebutting control has not finished), the routine proceeds to step 712,where it is determined whether the control mode determination value“mode” is equal to 0 or 3. The control mode determination value “mode”being “0” means “current supply off” (standby), and the control modedetermination value “mode” being “3” means “target position stopping andholding process.”

In this routine, a P-position-side butting control is performed when thecontrol mode determination value “mode” is equal to 0 or 3 so that it isperformed in a state that the rotor 32 is stopped at the P-position.

If the control mode determination value “mode” is equal to “0” (currentsupply off), the routine proceeds to step 713, where the count CT0 p ofa mode-0 time counter that counts the time during which the value “mode”is kept at “0” with the P-position is incremented. If the control modedetermination value “mode” is equal to “3” (target position stopping andholding process), the routine proceeds to step 714, where the count CT3p of a mode-3 time counter that counts the time during which the value“mode” is kept at “3” with the P-position is incremented. The above twotime counters are used for waiting until vibration of the rotor 32settles and the rotor 32 is stopped at the P-position.

If the control mode determination value “mode” is equal to “1” (ordinarydrive), “4” (reversing position stopping and holding process), or “5”(open-loop control), the rotor 32 is not stopped at the P-position andhence the routine proceeds to step 731, where the counts CT0 p and CT3 pof the respective time counters are cleared.

Then, at step 715, it is determined whether the count CT0 p of themode-0 time counter has exceeded a stop determination value K0 p or thecount CT3 p of the mode-3 time counter has exceeded a stop determinationvalue K3 p. The stop determination value K0 p corresponds to a time thatis necessary for settlement of vibration of the rotor 32 in a state thatthe shift position is the P-position and the control mode determinationvalue “mode” is equal to “0” (current supply off). The stopdetermination value K3 p corresponds to a time that is necessary forsettlement of vibration of the rotor 32 in a state that the shiftposition is the P-position and the control mode determination value“mode” is equal to “3” (target position stopping and holding process).

If the determination result at step 715 is “no,” it is determined thatvibration of the rotor 32 has not settled yet. The routine proceeds tostep 716, where the P-position-side butting control flag Xexp is set to“off.” In this case, a P-position-side butting control is not started.

On the other hand, if the determination result at step 715 is “yes,” itis determined that vibration of the rotor 32 has settled at theP-position. The routine proceeds to step 717, where the P-position-sidebutting control flag Xexp is set to “on.”

Then, the routine proceeds to step 718, where it is determined whetherthe P-position-side butting control flag Xexp is “on.” If the flag Xexpis “off,” the routine proceeds to step 719, where all of a buttingtarget count Ag, the count CTstop of a stop time counter, and the countCTg of a butting control time counter are set to “0.” Then this routineis finished.

On the other hand, if the P-position-side butting control flag Xexp is“on,” the routine proceeds to step 720, where the butting target countAg is set to a prescribed value Kgp. The butting target count Ag (Kgp)is set to such a value that a P-position-side butting control can causethe engaging portion 23 a of the detent spring 23 to reliably buttagainst the side wall of the P-position holding recess 24 of the detentlever 15.

If the P-position-side butting control flag Xexp is “on,” the targetcount Acnt is set to the target count Ag (i.e., Kgp) by a target countsetting routine (described later) shown in FIG. 45. Therefore, aP-position-side butting control is performed as shown in FIG. 41,whereby the engaging portion 23 a of the detent spring 23 butts againstthe side wall of the P-position holding recess 24 of the detent lever15.

During the P-position-side butting control, the count CTg of the buttingcontrol time counter is incremented at step 721 in FIG. 40 to measurethe execution time of the P-position-side butting control. At the nextstep 722, it is determined whether the execution time CTg of theP-position-side butting control has exceeded a maximum allowablelearning time Kg (e.g., 500 ms). If the P-position-side butting controlis performed normally, the engaging portion 23 a of the detent spring 23should butt against the side wall of the P-position holding recess 24 ofthe detent lever 15 in a time that is shorter than the maximum allowablelearning time Kg, whereupon learning of an encoder count Np at the timeof P-position-side butting (i.e., the P-position-side butting control)is finished.

Therefore, if the P-position-side butting control is not finished evenafter a lapse of the maximum allowable learning time Kg, some systemabnormality such as a failure in the SR motor 12 or the encoder 46should have happened. Therefore, in such a case, to finish theP-position-side butting control forcibly, the routine proceeds to step723, where the following settings are made: the P-position-side buttingcontrol flag Xexp=off, the P-position-side butting completion flagXp=off, the stop time count CTstop=0, the mode-3 time count CT3 p=0, themode-0 time count CT0 p=0, and the butting control time count CTg=0.Then, this routine is finished.

On the other hand, if the execution time CTg of the P-position-sidebutting control has not exceeded the maximum allowable learning time Kg,the routine proceeds to step 724, where a learned value Vp of theP-position output of the output shaft sensor 14 is updated according tothe following equation:Vp(i)=Vp(i−1)+{Vnsw−Vp(i−1)}/Rnswwhere Vp(i) is a current P-position output learned-value, Vp(i−1) is apreceding P-position output learned value, Vnsw is a current output ofthe output shaft sensor 14, and Rnsw is a smoothing coefficient.

At the next step 725, a learned value of the encoder count at the timeof P-position-side butting (herein after referred to as “P-position-sidebutting learned value”) GNp is updated according to the followingequation:GNp(i)=min{GNp(i−1),Ncnt}

In the above equation, GNp(i) is a current P-position-side buttinglearned value, GNp(i−1) is a preceding P-position-side butting learnedvalue, and Ncnt is a current encoder count. The function “min” is afunction of choosing a smaller one of GNp(i−1) and Ncnt. The encodercount Ncnt is set to a value that has been corrected by using an initialpositional deviation learned value Gcnt that was learned by the initialdrive routine of FIGS. 8 and 9.

Then, the routine proceeds to step 726, where whether a state that theengaging portion 23 a of the detent spring 23 is in contact with theside wall of the P-position-holding recess 24 of the detent lever 15 hasbeen established is determined on the basis of whether the currentP-position-side butting learned value GNp(i) is the same as thepreceding P-position-side butting learned value GNp(i−1).

As shown in FIG. 41, until a state that the engaging portion 23 a of thedetent spring 23 is in contact with the side wall of the P-positionholding recess 24 of the detent lever 15 is established, the rotor 32rotates and the encoder count Ncnt decreases gradually and hence thecurrent P-position-side butting learned value GNp(i) should be smallerthan the preceding P-position-side butting learned value GNp(i−1). Oncea state that the engaging portion 23 a of the detent spring 23 is incontact with the side wall of the P-position holding recess 24 of thedetent lever 15 is established afterwards, the rotor 32 stops rotatingand the encoder count Ncnt oscillates or no longer varies. Since thelearned value GNp(i) is no longer updated, the current P-position-sidebutting learned value GNp(i) should be greater than or equal to thepreceding P-position-side butting learned value GNp(i−1).

If the determination result at step 726 in FIG. 40 is “no,” a state thatthe engaging portion 23 a of the detent spring 23 is in contact with theside wall of the P-position holding recess 24 of the detent lever 15 isnot established and hence the routine proceeds to step 727, where thecount CTstop of the stop time counter is reset.

On the other hand, if the determination result at step 726 in FIG. 40 is“yes,” it is determined that the engaging portion 23 a of the detentspring 23 may have butted against the side wall of the P-positionholding recess 24 of the detent lever 15 and the routine proceeds tostep 728, where the count CTstop of the stop time counter that countsthe time of a contact state is incremented.

Then, the routine proceeds to step 729. The P-position-side buttingcontrol is continued until the count CTstop of the stop time counterexceeds a prescribed time Kstop (e.g., 60 ms) at step 729. At an instantwhen the state that the determination result at step 726 is “yes” hascontinued for the prescribed time Kstop or more, it is determined thatoccurrence of a contact state has been confirmed and the routineproceeds to step 730. At step 730, the following settings are made tofinish the P-position-side butting control: the P-position-side buttingcontrol flag Xexp=off, the P-position-side butting completion flagXp=on, the P-position-side butting encoder count Np=GNp+ΔNover (ΔNoveris a ride correction value), the stop time count CTstop=0, the mode-3time count CT3 p=0, and the mode-0 time count CT0 p=0. Then, thisroutine is finished.

The ride correction value ΔNover is an angle by which the engagingportion 23 a of the detent spring 23 slightly rides the side wall of theholding recess 24 or 25 of the detent lever 15 in a state that theengaging portion 23 a is in contact with the side wall during a buttingcontrol. The ride correction value ΔNover is set in accordance with abattery voltage as a power source voltage for the SR motor 12 by usingthe table of FIG. 37.

The angle (i.e., ride correction value ΔNover) by which the engagingportion 23 a of the detent spring 23 rides the side wall of the holdingrecess 24 or 25 of the detent lever 15 during a butting controlincreases as the torque of the SR motor 12 increases. In general, thetorque of the SR motor 12 varies with the power source voltage therefor(i.e., battery voltage) and hence a correlation exists between the powersource voltage and the torque of the SR motor 12. The power sourcevoltage can be used as replacement information for the torque of the SRmotor 12. Therefore, an actual measurement value of the movable range ofthe rotor 32 can be determined accurately by calculating an angle (i.e.,a ride correction value ΔNover) by which the engaging portion 23 a ofthe detent spring 23 rides the side wall of the holding recess 24 or 25of the detent lever 15 during a butting control on the basis of thepower source voltage (i.e., battery voltage) as replacement informationfor the torque of the SR motor 12 and correcting an actual measurementvalue of the movable range of the rotor 32 by the ride correction valueΔNover.

If the side wall of each of the holding recesses 24 and 25 of the detentlever 15 is formed so as not to be rideed by the engaging portion 23 aof the detent spring 23, it is not necessary to use the ride correctionvalue ΔNover and hence a relationship ΔNact=Nnp−Np holds.

[Non-P-Position-Side Butting Control]

The non-P-position-side butting control routine shown in FIGS. 42 and 43is a subroutine that is executed at step 703 of the play amount learningroutine of FIG. 36 when the instructed shift position is thenon-P-position. Upon activation of this routine, at step 741, it isdetermined whether the non-P-position-side butting completion flag Xnpis “on” (i.e., a non-P-position-side butting control has finished). If aP-position-side butting control has already finished, this routine isfinished without executing the remaining steps. As a result, anon-P-position-side butting control is performed only once during anignition switch on-period.

On the other hand, if it is determined at step 741 that thenon-P-position-side butting completion flag Xnp is “off” (anon-P-position-side butting control has not finished), the routineproceeds to step 742, where it is determined whether the control modedetermination value “mode” is equal to 0 (current supply off) or 3(target position stopping and holding process). In this routine, anon-P-position-side butting control is performed when the control modedetermination value “mode” is equal to 0 or 3 so that it is performed ina state that the rotor 32 is stopped at the non-P-position.

If the control mode determination value “mode” is equal to “0” (currentsupply off), the routine proceeds to step 743, where the count CT0 np ofa mode-0 time counter that counts the time during which the value “mode”is kept at “0” with the non-P-position is incremented. If the controlmode determination value “mode” is equal to “3” (target positionstopping and holding process), the routine proceeds to step 744, wherethe count CT3 np of a mode-3 time counter that counts the time duringwhich the value “mode” is kept at “3” with the non-P-position isincremented. The above two time counters are used for waiting untilvibration of the rotor 32 settles and the rotor 32 is stopped at theP-position.

If the control mode determination value “mode” is equal to “1” (ordinarydrive), “4” (reversing position stopping and holding process), or “5”(open-loop control), the rotor 32 is not stopped at the non-P-positionand hence the routine proceeds to step 761, where the counts CT0 np andCT3 np of the respective time counters are cleared.

Then, at step 745, it is determined whether the count CT0 np of themode-0 time counter has exceeded a stop determination value K0 np or thecount CT3 np of the mode-3 time counter has exceeded a stopdetermination value K3 np. The stop determination value K0 npcorresponds to a time that is necessary for settlement of vibration ofthe rotor 32 in a state that the shift position is the non-P-positionand the control mode determination value “mode” is equal to “0” (currentsupply off). The stop determination value K3 np corresponds to a timethat is necessary for settlement of vibration of the rotor 32 in a statethat the shift position is the non-P-position and the control modedetermination value “mode” is equal to “3” (target position stopping andholding process).

If the determination result at step 745 is “no,” it is determined thatvibration of the rotor 32 has not settled yet and the routine proceedsto step 746, where the non-P-position-side butting control flag Xexnp isset to “off.” In this case, a non-P-position-side butting control is notstarted.

On the other hand, if the determination result at step 745 is “yes,” itis determined that vibration of the rotor 32 has settled at thenon-P-position and the routine proceeds to step 747, where thenon-P-position-side butting control flag Xexnp is set to “on.”

Then, the routine proceeds to step 748, where it is determined whetherthe non-P-position-side butting control flag Xexnp is “on.” If the flagXexnp is “off,” the routine proceeds to step 749, where all of thebutting target count Ag, the count CTstop of the stop time counter, andthe count CTg of the butting control time counter are set to “0.” Thenthis routine is finished.

On the other hand, if the non-P-position-side butting control flag Xexnpis “on,” the routine proceeds to step 750, where the butting targetcount Ag is set to a prescribed value Kgnp. The butting target count Ag(Kgnp) is set to such a value that an on-P-position-side butting controlcan cause the engaging portion 23 a of the detent spring 23 to reliablybutt against the side wall of the non-P-position holding recess 25 ofthe detent lever 15.

If the non-P-position-side butting control flag Xexnp is “on,” thetarget count Acnt is set to the target count Ag (i.e., Kgnp) by a targetcount setting routine (described later) shown in FIG. 45. Therefore, anon-P-position-side butting control is performed, whereby the engagingportion 23 a of the detent spring 23 butts against the side wall of thenon-P-position holding recess 25 of the detent lever 15.

During the non-P-position-side butting control, the count CTg of thebutting control time counter is incremented at step 751 in FIG. 43 tomeasure the execution time of the non-P-position-side butting control.At the next step 752, it is determined whether the execution time CTg ofthe non-P-position-side butting control has exceeded the maximumallowable learning time Kg (e.g., 500 ms). If the non-P-position-sidebutting control is performed normally, the engaging portion 23 a of thedetent spring 23 should butt against the side wall of the P-positionholding recess 24 of the detent lever 15 in a time that is shorter thanthe maximum allowable learning time Kg, whereupon learning of an encodercount Nnp at the time of non-P-position-side butting (i.e., thenon-P-position-side butting control) is finished.

Therefore, if the non-P-position-side butting control is not finishedeven after a lapse of the maximum allowable learning time Kg, somesystem abnormality such as a failure in the SR motor 12 or the encoder46 should have happened. Therefore, in such a case, to finish thenon-P-position-side butting control forcibly, the routine proceeds tostep 753, where the following settings are made: the non-P-position-sidebutting control flag Xexnp=off, the non-P-position-side buttingcompletion flag Xnp=off, the stop time count CTstop=0, the mode-3 timecount CT3 np=0, the mode-0 time count CT0 np=0, and the butting controltime count CTg=0. Then, this routine is finished.

On the other hand, if the execution time CTg of the non-P-position-sidebutting control has not exceeded the maximum allowable learning time Kg,the routine proceeds to step 754, where a learned value Vnp of thenon-P-position output of the output shaft sensor 14 is updated accordingto the following equation:Vnp(i)=Vnp(i−1)+{Vnsw−Vnp(i−1)}/Rnswwhere Vnp(i) is a current non-P-position output learned value, Vnp(i−1)is a preceding non-P-position output learned value, Vnsw is a currentoutput of the output shaft sensor 14, and Rnsw is a smoothingcoefficient.

At the next step 755, a learned value of the encoder count at the timeof non-P-position-side butting (hereinafter referred to as“non-P-position-side butting learned value”) GNnp is updated accordingto the following equation:GNnp(i)=max{GNnp(i−1),Ncnt}

In the above equation, GNnp(i) is a current non-P-position-side buttinglearned value, GNnp(i−1) is a preceding non-P-position-side buttinglearned value, and Ncnt is a current encoder count. The function “max”is a function of choosing a larger one of GNnp(i−1) and Ncnt. Theencoder count Ncnt is set to a value that has been corrected by using aninitial positional deviation learned value Gcnt that was learned by theinitial drive routine of FIGS. 8 and 9.

Then, the routine proceeds to step 756, where whether a state that theengaging portion 23 a of the detent spring 23 is in contact with thesidewall of then on-P-position holding recess 25 of the detent lever 15has been established is determined on the basis of whether the currentnon-P-position-side butting learned value GNnp(i) is the same as thepreceding non-P-position-side butting learned value GNnp(i−1).

Until a state that the engaging portion 23 a of the detent spring 23 isin contact with the side wall of the non-P-position holding recess 25 ofthe detent lever 15 is established, the rotor 32 rotates and the encodercount Ncnt increases gradually and hence the current non-P-position-sidebutting learned value GNnp(i) should be greater than the precedingnon-P-position-side butting learned value GNnp(i−1). Once a state thatthe engaging portion 23 a of the detent spring 23 is in contact with theside 2 c wall of the non-P-position holding recess 25 of the detentlever 15 is established afterwards, the rotor 32 stops rotating and theencoder count Ncnt oscillates or no longer varies. Since the learnedvalue GNnp(i) is no longer updated, the current non-P-position-sidebutting learned value GNnp(i) should be greater than or equal to thepreceding non-P-position-side butting learned value GNnp(i−1).

If the determination result at step 756 in FIG. 43 is “no,” a state thatthe engaging portion 23 a of the detent spring 23 is in contact with theside wall of the non-P-position holding recess 25 of the detent lever 15is not established and hence the routine proceeds to step 757, where thecount CTstop of the stop time counter is reset.

On the other hand, if the determination result at step 756 is “yes,” itis determined that the engaging portion 23 a of the detent spring 23 mayhave butted against the side wall of the non-P-position holding recess25 of the detent lever 15 and the routine proceeds to step 758, wherethe count CTstop of the stop time counter that counts the time of acontact state is incremented.

Then, the routine proceeds to step 759. The non-P-position-side buttingcontrol is continued until the count CTstop of the stop time counterexceeds the prescribed time Kstop (e.g., 60 ms) at step 759. At aninstant when the state that the determination result at step 756 is“yes” has continued for the prescribed time Kstop or more, it isdetermined that occurrence of a contact state has been confirmed and theroutine proceeds to step 760. At step 760, the following settings aremade to finish the non-P-position-side butting control: thenon-P-position-side butting control flag Xexnp=off, thenon-P-position-side butting completion flag Xnp=on, thenon-P-position-side butting encoder count Nnp=GNnp−ΔNover (ΔNover is theride correction value), the stop time count CTstop=0, the mode-3 timecount CT3 np=0, and the mode-0 time count CT0 np=0. Then, this routineis finished.

FIG. 44 is a time chart showing exemplary execution timing of aP-position-side butting control and a non-P-position-side buttingcontrol. In the example of FIG. 44, when the instructed shift positionis switched from the P-position to the non-P-position sometime after anon-manipulation on the ignition switch (i.e., application of power), thetarget count Acnt is set to a temporary target count (e.g., 18°) for thenon-P-position. The temporary target count (e.g., 18°) corresponds to arotation angle at which the engaging portion 23 a of the detent spring23 has reliably gone over the projection between the holding recesses 24and 25 of the detent lever 15.

Therefore, after being rotated to a position corresponding to thetemporary target count (e.g., 18°) by a feedback control, the rotor 32is rotated to the non-P-position target position (i.e., the bottom ofthe non-P-position holding recess 25) by utilizing momentum of theengaging portion 23 a of the detent spring 23 being dropped toward thebottom along the inclined side wall of the non-P-position holding recess25 of the detent lever 15 by elastic force of the detent spring 23.

Then, when the non-P-position-side butting control flag Xexnp is set to“on,” the target count Acnt is set to a non-P-position-side buttingtarget count Ag (i.e., Kgnp) and a non-P-position-side butting controlis started. An encoder count Nnp (i.e., GNnp) is learned when theengaging portion 23 a of the detent spring 23 butts against the sidewall of the non-P-position holding recess 25 of the detent lever 15.After the learning of the encoder count Nnp, the non-P-position-sidebutting control flag Xexnp is set to “off” and the non-P-position-sidebutting control is finished.

Then, when the instructed shift position is switched from thenon-P-position to the P-position, the target count Acnt is set to atemporary target count (e.g., 0°) for the P-position. After beingrotated to a position corresponding to the temporary target count (e.g.,0°) by a feedback control, the rotor 32 is rotated to the P-positiontarget position (i.e., the bottom of the P-position holding recess 24)by utilizing momentum of the engaging portion 23 a of the detent spring23 being dropped toward the bottom along the inclined side wall of theP-position holding recess 24 of the detent lever 15 by elastic force ofthe detent spring 23.

Then, when the P-position-side butting control flag Xexp is set to “on,”the target count Acnt is set to a non-P-position-side butting targetcount Ag (i.e., Kgp) and a P-position-side butting control is started.An encoder count Np (i.e., GNp) is learned when the engaging portion 23a of the detent spring 23 butts against the side wall of the P-positionholding recess 24 of the detent lever 15. After the learning of theencoder count Np, the P-position-side butting control flag Xexp is setto “off” and the P-position-side butting control is finished.

[Target Count Setting]

The target count setting routine shown in FIGS. 45 and 46 is executed ina prescribed cycle (e.g., every 8 ms) after the end of an initial drive.Upon activation of this routine, at step 771, it is determined whether abutting control is being performed (i.e., whether the P-position-sidebutting control flag Xexp or the non-P-position-side butting controlflag Xexnp is “on”). If a butting control is being performed, theroutine proceeds to step 772, where the target count Acnt is set to abutting target count Ag. The butting target count Ag is set at step 720in FIG. 39 or step 750 in FIG. 42.

On the other hand, if no butting control is being performed, the routineproceeds to step 773, where it is determined whether an instructed shiftposition sft that is stored in the RAM of the ECU 41 is the P-position.If it is the P-position, the routine proceeds to step 774, where it isdetermined whether a P-position-side butting control has completed(i.e., whether the P-position-side butting completion flag Xp is “on”).If a P-position-side butting control has completed, the routine proceedsto step 775, a P-position target count Acnt is calculated according tothe following equation:Acnt=Np+ΔGp

In the above equation, Np is an encoder count at the time ofP-position-side butting and is a value GNp that was learned by theP-position-side butting control routine of FIGS. 39 and 40. And ΔGp is alearned value of the P-position-side play amount, and is a precedingvalue that is stored in the nonvolatile memory of the ECU 41 until it isupdated by a current (i.e., newly calculated) learned value ΔGp of theplay amount at step 706 of the play amount learning routine of FIG. 36.

On the other hand, if a P-position-side butting control has notcompleted, the determination result at step 774 is “no” and hence theroutine proceeds to step 776, where it is determined whether anon-P-position-side butting control has completed (i.e., whether thenon-P-position-side butting completion flag Xnp is “on”). If anon-P-position-side butting control has completed, the routine proceedsto step 777, where a P-position target count Acnt is calculatedaccording to the following equation:Acnt=Nnp−ΔNact+ΔGp

In the above equation, Nnp is an encoder count at the time ofnon-P-position-side butting and is a value GNnp that was learned by thenon-P-position-side butting control routine of FIGS. 42 and 43. AndΔNact is an actual measurement value of the movable range of the rotor32, and is a preceding value that is stored in the nonvolatile memory ofthe ECU 41 until it is updated by a current (i.e., newly calculated)actual measurement value ΔNact of the movable range at step 705 of theplay amount learning routine of FIG. 36.

If neither a P-position-side butting control nor a non-P-position-sideone has completed (i.e., both of the P-position-side andnon-P-position-side butting completion flags Xp and Xnp are “off”), thetarget count Acnt cannot be corrected by using a play amount ΔGp or ΔGnpbecause neither an encoder count Np at the time of P-position sidebutting nor an encoder count Nnp at the time of non-P-position sidebutting has been learned. Therefore, in this case, the routine proceedsto step 778, where the P-position target count Acnt is set to atemporary P-position target count “0.”

On the other hand, if it is determined at step 773 that the instructedshift position sft is the non-P-position, the routine proceeds to step779 in FIG. 46, where it is determined whether a-non-P-position-sidebutting control has completed (i.e., whether then on-P-position-sidebutting completion flag Xnp is “on”). If a non-P-position-side buttingcontrol has completed the routine proceeds to step 780, where anon-P-position target count Acnt is calculated according to thefollowing equation:Acnt=Nnp−ΔGnp

In the above equation, ΔGnp is a learned value of thenon-P-position-side play amount, and is a preceding value that is storedin the nonvolatile memory of the ECU 41 until it is updated by a current(i.e., newly calculated) learned value ΔGnp of the play amount at step706 of the play amount learning routine of FIG. 36.

On the other hand, if a non-P-position-side butting control has notcompleted, the determination result at step 779 is “no” and hence theroutine proceeds to step 781, where it is determined whether aP-position-side butting control has completed (i.e., whether theP-position-side butting completion flag Xp is “on”). If aP-position-side butting control has completed, the routine proceeds tostep 782, where a non-P-position target count Acnt is calculatedaccording to the following equation:Acnt=Np+ΔNact−ΔGnp

If neither a P-position-side butting control nor a non-P-position-sideone has completed (i.e., both of the P-position-side andnon-P-position-side butting completion flags Xp and Xnp are “off”), theroutine proceeds to step 783, where the non-P-position target count Acntis set to a temporary non-P-position target count Knotp “e.g., 18.5°.”

In setting a target count Acnt in this routine, preceding values storedin the nonvolatile memory of the ECU 41 are used as play amount learnedvalues ΔGp and ΔGnp and a movable range actual measurement value ΔNactuntil the former are updated. Alternatively, the target count Acnt maybe set to a temporary target count (0 or Knotp) until values stored inthe nonvolatile memory are updated.

[Setting of Instructed Shift Position]

An instructed shift position setting routine shown in FIG. 47 isactivated in a prescribed cycle (e.g., every 8 ms). Upon activation ofthis routine, at step 801, it is determined whether the present instantbelongs to a starting period (i.e., the present instant is immediatelyafter resetting of the CPU of the ECU 41 or application of power to it).If the present instant belongs to a starting period, the routineproceeds to step 806, where an actual shift position that is detectedfrom an output Vnsw of the output shaft sensor 14 is set as aninstructed shift position of starting. Then this routine is finished.

On the other hand, if it is determined at step 801 that present instantdoes not belong to a starting period, the routine proceeds to step 802,where it is determined whether the P-position switch 43 has beenmanipulated. If the P-position switch 43 has been manipulated, theroutine proceeds to step 803, where the instructed shift position sft isset to the P-position.

If the P-position switch 43 has not been manipulated, the routineproceeds to step 804, where it is determined whether the non-P-positionswitch 44 has been manipulated. If the non-P-position switch 44 has beenmanipulated, the routine proceeds to step 805, where the instructedshift position sft is set to the non-P-position.

Next, a method for setting an instructed shift position sft will bedescribed by using a time chart of FIG. 48. FIG. 48 shows an exemplarycontrol that is performed when the CPU of the ECU 41 is reset for acertain reason in a state that the vehicle is being driven with theinstructed shift position sft set to the non-P-position. If the CPU ofthe ECU 41 is reset for a certain reason while the vehicle is running,it is determined immediately thereafter that the present instant belongsto a starting period and an actual shift position detected from anoutput Vnsw of the output shaft sensor 14 is set as an instructed shiftposition sft. Since the actual shift position is the non-P-positionwhile the vehicle is running, the instructed shift position sft is setto the non-P-position on the basis of an output Vnsw of the output shaftsensor 14 in a starting period after the CPU of the ECU 41 has beenreset for a certain reason.

As described above, even if the CPU of the ECU 41 is reset for a certainreason while the vehicle is running, the instructed shift position sftis not changed in association with the resetting and remains thenon-P-position. This prevents an undesirable event that the shiftposition is switched to the P-position contrary to the intention of adriver.

Except during a starting period, the instructed shift position sft isswitched to the P-position or the non-P-position in response to amanipulation on the P-position switch 43 or the non-P-position switch44.

In the above embodiment, in an initial drive that is performed afterapplication of power to the ECU 41, current supply is effected for allthe phases of the SR motor 12 around according to a prescribed timeschedule. Therefore, a rotation position of the rotor 32 necessarilycoincides with one of the current supply phases. From this time onward,the rotor 32 rotates in synchronism with the current supply phaseswitching and an A-phase signal and a B-phase signal are output from theencoder 46 in synchronism with the rotation of the rotor 32. Both ofrising edges and trailing edges of the A-phase signal and the B-phasesignal of the encoder 46 are counted during the initial drive.Therefore, an angle (i.e., a rotation amount) by which the rotor 32 hasrotated actually in synchronism with the current supply phase switchinguntil the end of the initial drive is recognized by checking an encodercount at the end of the initial drive, whereby a correspondingrelationship between the encoder count, a rotation position of the rotor32, and a current supply phase at the end of the initial drive can berecognized.

Therefore, in this embodiment, an encoder count at the end of an initialdrive is learned as an initial positional deviation learned value andeach encoder count is corrected by using the initial positionaldeviation learned value in a subsequent ordinary drive. Therefore, adeviation between an encoder count and a current supply phase (i.e., arotation position of the rotor 32) at the end of an initial drive can becorrected for and correct current supply phases can be selected duringan ordinary drive.

Further, in this embodiment, one-phase current supply and two-phasecurrent supply are performed alternately in an initial drive. Therefore,the rotor rotation angle per step (i.e., per current supply) is made ahalf of that of the one-phase current supply method in which currentsupply is always effected for only one phase and the two-phase currentsupply method in which current supply is always effected for two phases,whereby the rotation position of the rotor 32 and the current supply canreliably be synchronized with each other during an initial drive.Further, output signals of the encoder 46 can be stabilized by stoppingvibration of the rotor 32 by two-phase current supply which produceshigh torque.

Even after the rotation position of the rotor 32 and the current supplyhave been synchronized with each other during an initial drive, therotor 32 still vibrates with one-phase current supply which produces lowtorque. In view of this, in this embodiment, during an initial drive,the one-phase current supply time is set shorter than the two-phasecurrent supply time. Shortening the time of one-phase current supplywhich causes vibration of the rotor 32 and making switching to the nexttwo-phase current supply as soon as possible makes it possible tostabilize output signals of the encoder 46 by stopping vibration of therotor 32 quickly and to shorten the initial drive time.

However, in the invention, the one-phase current supply time is set thesame as the two-phase current supply time. The intended object of theinvention can be attained even in such a case.

Although in this embodiment an initial drive is started with one-phasecurrent supply, it may be started with two-phase current supply. In thelatter case, even if a position of the rotor 32 at a start of an initialdrive does not correctly coincide with a position corresponding to firsttwo-phase current supply, the rotation position of the rotor 32 and thecurrent supply phase can be synchronized with each other from the startof the initial drive as long as the rotor 32 is located in such a rangeas to be moved by high torque of the two-phase current supply.

The invention is not limited to the configuration that one-phase currentsupply and two-phase current supply are performed alternately in aninitial drive; an initial drive may be performed by the one-phasecurrent supply method in which current supply is always effected foronly one phase or the two-phase current supply method in which currentsupply is always effected for two phases.

As described above, producing high torque, two-phase current supply canrotate the rotor 32 to a position corresponding to the two-phase currentsupply even if the rotor 32 is deviated from the position correspondingto the two-phase current supply. Therefore, the probability thateffecting two-phase current supply only once or twice during an initialdrive synchronizes the rotation position of the rotor 32 and the currentsupply phase with each other would be high.

In view of the above, in this embodiment, if a manipulation of switchingthe range from the P-position to the non-P-position is performed duringa P-position initial drive, a transition to an ordinary drive aftercompletion of the next two-phase current supply (after completion oftwo-phase current supply itself if a position switching manipulation isperformed during the two-phase current supply) and a correspondingrelationship between an encoder count, a rotation position of the rotor32, and a current supply phase at the end of the two-phase currentsupply is learned. Therefore, when a manipulation of switching the rangefrom the P-position to the non-P-position is performed during an initialdrive, switching from the P-position to the non-P-position can be madequickly in such a manner that a corresponding relationship between anencoder count, a rotation position of the rotor 32, and a current supplyphase is learned and then a transition to an ordinary drive is madequickly. This prevents the driver from feeling incongruous.

Usually, the SR motor 12 is not started with the non-P-position (becausethe ignition switch cannot be turned off with a range other than theP-position). However, under a special condition, there may occur a casethat the SR motor 12 is started with then on-P-position. For example,the SR motor 12 may be started with the non-P-position in a case thatthe power to the ECU 41 is shut off instantaneously while the vehicle isrunning or the vehicle is maintained or inspected in a service station.

A logic for starting the SR motor 12 with the non-P-position may beomitted to simplify the control specification. However, in thisembodiment, to accommodate the case that an initial drive is performedwith the non-P-position, if a manipulation of switching the range fromthe P-position to the non-P-position during a P-position initial drive,switching from the P-position to the non-P-position is made in such amanner a transition to an ordinary drive is made after the initial driveis completed as scheduled.

Naturally, in the invention, even in the case that an initial drive isperformed with the non-P-position, a procedure may be followed that islike the one employed in the case that an initial drive is performedwith the P-position. That is, when a position switching manipulation isperformed, a transition is made to an ordinary drive after completion ofthe next two-phase current supply (after completion of two-phase currentsupply itself if a position switching manipulation is performed duringthe two-phase current supply) and a corresponding relationship betweenan encoder count, a rotation position of the rotor 32, and a currentsupply phase at the end of the two-phase current supply is learned.

As a further alternative, even if a manipulation of switching the rangefrom the P-position to the non-P-position is performed during aP-position initial drive, switching from the P-position to thenon-P-position may be made in such a manner that a transition is made toan ordinary drive after the initial drive is completed as scheduled(priority is given to learning of an initial positional deviation).

In the above embodiment, in a feedback control, the variation directionof the encoder count is monitored and it is determined whether therotation direction of the rotor 32 has reversed contrary to a rotationdirection instruction (i.e., an instruction of a rotation directiontoward a target position). If reversing of the rotation direction isdetected, current supply for phases with which the rotor 32 is driven inthe reverse direction is prohibited and current supply is effected forA-phase for which current supply was effected immediately before thereversing. This makes it possible to generate, upon detection ofrotation reversing, braking torque that suppresses the reverse rotationof the rotor 32. The braking torque can suppress the reverse rotationquickly, whereby the rotation direction of the rotor 32 can quickly bereturned from the reverse direction to the normal direction. A stableposition switching control is enabled.

In this embodiment, upon detection of rotation reversing, current supplyis effected for A-phase for which current supply was effectedimmediately before the reversing. Alternatively, current supply for anyphase may be prohibited upon detection of rotation reversing. Also inthis case, generation of torque in the reverse rotation direction can beprevented and hence the intended object of the invention can beattained.

In the above embodiment, as shown in FIG. 18, to start a feedbackcontrol from a state that the SR motor 12 is not energized, a feedbackcontrol start position stopping and holding process in which a currentsupply phase is selected on the basis of a current encoder count andcurrent supply is effected for that phase is executed for a prescribedtime (e.g., 10 ms) and the rotor 32 is thereafter rotated to a targetposition by performing a feedback control. Therefore, even if theposition of the rotor 32 deviated in a period when the rotor 32 shouldbe stopped, the rotor 32 can be positioned at a proper position at thestart of a feedback control.

As a result, at the start of the feedback control, the position of therotor 32 and the current supply phase (i.e., encoder count) can reliablybe synchronized with each other from the first current supply phase. Itbecomes possible to prevent a loss of synchronization and prevent therotor 32 from rotating away from a target position at the start of thefeedback control, to allow the rotor 32 to reliably rotate to the targetposition with a stable feedback control, and to perform a positionswitching control (positioning control) that is highly stable andreliable.

Further, in this embodiment, after the end of a feedback control,current supply of the SR motor 12 is turned off after a target positionstopping and holding process in which current supply phase is selectedso that the rotor 32 is stopped and kept at a position where the rotor32 existed at the end of the feedback control and current supply iseffected for that phase is executed for a prescribed time (e.g., 50 ms).This prevents the rotor 32 from passing a target position due to inertiaat the end of a feedback control (occurrence of an overshoot). Therequirement of increase in the response speed of the position switchingcontrol can be satisfied while high stability and reliability of theposition switching control are maintained.

In this embodiment, when the instructed shift position (target position)is changed during a feedback control to necessitate reversing therotation direction of the rotor 32, a reversing position stopping andholding process in which current supply phase with which to stop andhold the rotor 32 is selected and current supply is effected for thatphase is executed for a prescribed time (e.g., 50 ms). Then, a feedbackcontrol is restarted to rotate the rotor 32 to a new target position.Therefore, when the instructed shift position (target position) ischanged during a feedback control, a reversing operation can beperformed stably by determining a reversing position of the rotor 32.This prevents a loss of synchronization (i.e., deviation of currentsupply phases) due to deviation of the reversing position and therebymakes it possible to reliably rotate the rotor 32 to a new targetposition.

In this embodiment, a feedback control is finished and a transition ismade to a target position stopping and holding process when thedifference between the encoder count and a target count corresponding toa target position has become smaller than or equal to a prescribed value(e.g., a count corresponding to A-phase lead of the current supplyphase) during the feedback control. Therefore, the last current supplyphase of the R feedback control coincides with a current supply phasewith which to stop and keep the rotor 32 at the target position. Thismakes it possible to continue to effect current supply for the lastcurrent supply phase of the feedback control even after the transitionto the target position stopping and holding process. The transition tothe target position stopping and holding process from the feedbackcontrol can be made smoothly.

In this embodiment, the current supply method by which a feedbackcontrol start position stopping and holding process, a target positionstopping and holding process, or a reversing position stopping andholding process is going to be executed is one-phase current supply, itis corrected to two-phase current supply. Therefore, each of the abovestopping and holding processes can be executed by two-phase currentsupply which produces high torque. This provides an advantage thatvibration of the rotor 32 can be prevented and the rotor 32 can reliablybe stopped and kept at an intended position.

However, in the invention, each of the above stopping and holdingprocesses may be executed by one-phase current supply. Even in thiscase, the effect of preventing position deviation of the rotor 32 can beobtained.

In this embodiment, the rotor 32 is driven by the one/two-phase currentsupply method in which one-phase current supply and two-phase currentsupply are performed alternately. Alternatively, the one-phase currentsupply method in which the rotor 32 is driven by only one-phase currentsupply or the two-phase current supply method in which the rotor 32 isdriven by only two-phase current supply may be employed.

In the above embodiment, a play amount of the transmission system of theposition switching mechanism 11 is learned. Therefore, even if therotation transmission system has play amount, a target position (i.e.,target count) can be set with the play amount taken into consideration.This makes it possible to accurately control the manipulated variablefor the position switching mechanism 11.

Further, in this embodiment, in a state that the rotor 32 is stopped atthe P-position after a control on the position switching mechanism 11was started without performing play amount learning, a P-position-sidebutting control is performed and an encoder count at the time ofP-position-side butting is stored in the RAM of the ECU 41. In a statethat the rotor 32 is stopped at the non-P-position, anon-P-position-side butting control is performed and an encoder count atthe time of non-P-position-side butting is stored in the RAM of the ECU41. The difference between the encoder count at the time ofP-position-side butting and the encoder count at the time ofnon-P-position-side butting is calculated as an actual measurement valueof the movable range of the position switching mechanism 11. Thedifference between the actual measurement value of the movable range andits design value is learned as a play amount of the rotationtransmission system.

Therefore, even if there was sufficient time to learn a play amount ofthe rotation transmission system from application of power to the ECU 41to a start of a control on the position switching mechanism 11 and thecontrol on the position switching mechanism 11 was started withoutperforming play amount learning, a play amount of the rotationtransmission system can be learned afterwards by performing each buttingcontrol when the rotor 32 is stopped at the P-position or thenon-P-position.

In this embodiment, during a butting control, the current supply dutyratio (i.e., current supply factor) of the SR motor 12 is set smallerthan in an ordinary drive, whereby the torque of the SR motor 12 is madelower than in an ordinary drive. This makes it possible to weaken theforce of causing the engaging portion 23 a of the detent spring 23 tobutt against the side wall of the holding recess 24 or 25 of the detentlever 15 during a butting control. As a result, the parts of therotation transmission system such as the engaging portion 23 a of thedetent spring 23 are prevented from being deformed or damaged due tobutting controls, whereby high durability and reliability can besecured.

In this embodiment, the rotation speed of the rotor 32 is lowered duringa butting control. This decreases the speed at which the engagingportion 23 a of the detent spring 23 collides with the side wall of theholding recess 24 or 25 of the detent lever 15 during butting controls,and hence can weaken resulting impact. With the above effect of thereduction in the torque of the SR motor 12, this measure makes itpossible to more reliably prevent the parts of the rotation transmissionsystem such as the engaging portion 23 a of the detent spring 23 frombeing deformed or damaged, to thereby increase the durability andreliability.

In this embodiment, during a butting control, an angle (i.e., a ridecorrection value) by which the engaging portion 23 a of the detentspring 23 slightly rides the side wall of the holding recess 24 or 25 ofthe detent lever 15 in a state that the engaging portion 23 a is incontact with the side wall is inferred on the basis of the batteryvoltage as a power source voltage for the SR motor 12. And an actualmeasurement value of the movable range of the position switchingmechanism 11 is corrected by the inferred value. Therefore, even if theposition switching mechanism 11 is configured in such a manner that theengaging portion 23 a of the detent spring 23 rides the side wall of theholding recess 24 or 25 of the detent lever 15 during a butting control,an actual measurement value of the movable range of the positionswitching mechanism 11 can be determined accurately.

In this embodiment, when the butting learned value has come not to beupdated during a butting control, it is determined that a state that theengaging portion 23 a of the detent spring 23 is in contact with theside wall of the holding recess 24 or 25 of the detent lever 15 has beenestablished, whereupon the butting control is finished. Therefore, anencoder count corresponding to a state that the engaging portion 23 a ofthe detent spring 23 is in contact with the side wall of the holdingrecess 24 or 25 of the detent lever 15 has been established can bedetermined correctly, and hence an actual measurement value of themovable range of the position switching mechanism 11 can be determinedaccurately. Further, the period during which the engaging portion 23 aof the detent spring 23 is kept in contact with the side wall of theholding recess 24 or 25 of the detent lever 15 can be shortened, wherebythe degree of deterioration of the parts of the rotation transmissionsystem such as the engaging portion 23 a of the detent spring 23 due tobutting controls can further be reduced.

However, in the invention, the butting control may always be performedfor a constant time. Even in this case, the degree of deterioration ofthe parts of the rotation transmission system can be reduced byweakening the torque of the SR motor 12 during butting controls.

In this embodiment, a butting control is finished when it is determinedthat the engaging portion 23 a of the detent spring 23 does not buttagainst the side wall of the holding recess 24 or 25 of the detent lever15 even if the butting control has been performed for a prescribed timeor more. This makes it possible to finish a butting control forciblywhen a certain abnormality such as a failure in the SR motor 12 or theencoder 46 has occurred, which is a fail-safe measure.

In the above embodiment, during a feedback control, the phase lead ofthe current supply phase is corrected in accordance with the rotationspeed of the rotor 32. Therefore, the starting torque can be increasedby decreasing the phase lead of the current supply phase at a start of afeedback control, whereby the rotation speed of the rotor 32 can beraised quickly after the feedback control is started. Since the phaselead of the current supply phase is corrected so as to increase as therotation speed of the rotor 12 increases, a synchronized relationshipbetween the generation of torque of the current supply phase and theactual rotation phase of the rotor 32 can be maintained even duringhigh-speed rotation, which makes it possible to rotate the rotor 32stably at high speed.

When the rotor 32 has come close to a target position afterwards, thephase lead of the current supply phase is corrected in the decreasingdirection. This establishes a state that the generation of torque of thecurrent supply phase delays from the actual rotation phase of the rotor32, which makes it possible to decrease the driving torque or generatetorque (braking torque) in the direction opposite to the rotationdirection of the rotor 32, whereby the rotation speed of the rotor 32can be decreased reliably and the rotor 32 can be stopped at the targetposition correctly. This makes it possible to perform a feedback controlthat is superior in starting performance, high-speed rotationperformance, and stop performance (deceleration performance).

In this embodiment, as shown in FIG. 18, to start a feedback controlfrom a state that the SR motor 12 is not energized, a feedback controlstart position stopping and holding process in which current supplyphases are selected on the basis of a current encoder count and therotor 32 is stopped and kept at a stop position by two-phase currentsupply is executed for a prescribed time (e.g., 10 ms). The feedbackcontrol is then performed to rotate the rotor 32 to a target position.With this measure, even if the rotor 32 deviated in a period when itshould be stopped, the rotor 32 can be positioned at a proper positionat the start of a feedback control.

As a result, at the start of a feedback control, the position of therotor 32 and the current supply phase (i.e., encoder count) can reliablybe synchronized with each other from the first current supply phase. Itbecomes possible to prevent a loss of synchronization and prevent therotor 32 from rotating away from a target position at the start of afeedback control, to allow the rotor 32 to reliably rotate to a targetposition with a stable feedback control, and to perform a positionswitching control (positioning control) that is highly stable andreliable.

Further, in this embodiment, after the end of a feedback control,current supply of the SR motor 12 is turned off after a target positionstopping and holding process in which the rotor 32 is stopped and kept,by two-phase current supply, at a position where the rotor 32 existed atthe end of the feedback control is executed for a prescribed time (e.g.,50 ms). This prevents the rotor 32 from passing a target position due toinertia at the end of a feedback control (occurrence of an overshoot).The requirement of increase in the response speed of the positionswitching control can be satisfied while high stability and reliabilityof the position switching control are maintained.

In this embodiment, when the instructed shift position (target position)is changed during a feedback control to necessitate reversing therotation direction of the rotor 32, a reversing position stopping andholding process in which the rotor 32 is stopped and kept at a reversingposition by two-phase current supply is executed for a prescribed time(e.g., 50 ms). Then, a feedback control is restarted to rotate the rotor32 to a new target position. Therefore, when the instructed shiftposition (target position) is changed during a feedback control, areversing operation can be performed stably by determining a reversingposition of the rotor 32. This prevents a loss of synchronization (i.e.,deviation of current supply phases) due to deviation of the reversingposition and thereby makes it possible to reliably rotate the rotor 32to a new target position.

In this embodiment, the current supply method by which a feedbackcontrol start position stopping and holding process, a target positionstopping and holding process, or a reversing position stopping andholding process is going to be executed is one-phase current supply, itis corrected to two-phase current supply (U and V-phases, V andW-phases, or U and W-phases). Therefore, each of the above stopping andholding processes can be executed by two-phase current supply whichproduces high torque. This provides an advantage that vibration of therotor 32 can be prevented and the rotor 32 can reliably be stopped andkept at an intended position.

In this embodiment, the rotor 32 is driven by the one/two-phase currentsupply method in which one-phase current supply and two-phase currentsupply are performed alternately. Alternatively, the one-phase currentsupply method in which the rotor 32 is driven by only one-phase currentsupply or the two-phase current supply method in which the rotor 32 isdriven by only two-phase current supply may be employed.

In an open-loop control, processing of stopping and holding the rotor 32at a start of driving, at an end of driving, or in changing the targetposition (i.e., in reversing the rotation direction) may be performed bytwo-phase current supply which produces higher torque than one-phasecurrent supply does. With this measure, also in an open-loop control,the rotor 32 can be stopped and held stably at a start of driving, at anend of driving, or in changing the target position (i.e., in reversingthe rotation direction). The stability of an open-loop control can thusbe increased.

Second Embodiment

In the above first embodiment, with attention paid to the fact that theoutput shaft sensor 14 for detecting a rotation position of the outputshaft 13 of the SR motor 12 (i.e., an actual shift position) isprovided, an actual shift position that is detected by the output shaftsensor 14 at starting (e.g., after resetting of the CPU of the ECU 41 orafter power application to it) is set as an instructed shift positionsft of starting. Where the output shaft sensor 14 is not provided, it isappropriate to set an instructed shift position sft of starting as in asecond embodiment of the invention shown in FIGS. 49 and 50. It goeswithout saying that the instructed shift position setting methodaccording to the second embodiment may be applied to a system that isprovided with the output shaft sensor 14.

In the second embodiment, a latest instructed shift position sft isstored in the nonvolatile memory such as an SRAM of the ECU 41 while thevehicle is driven. At starting (e.g., after resetting of the CPU of theECU 41 or power application to it), an instructed shift position storagevalue SRAMsft of the nonvolatile memory is set as an instructed shiftposition sft of starting.

If the battery is removed, the instructed shift position storage valueSRAMsft disappears from the nonvolatile memory such as an SRAM whichneeds to be backed up by the battery while the power is off. In view ofthis, in the second embodiment, when removal of the battery is detected,the instructed shift position sft of starting is set to the P-position,for example, that is a shift position with which work of removing thebattery is performed.

If the battery is removed, a butting learned values GNp etc. that arestored in the nonvolatile memory also disappear. Therefore, when removalof the battery is detected, not only is the instructed shift positionsft of starting set to the P-position but also a P-position-side buttingcontrol is performed forcibly, whereby a P-position-side butting learnedvalue GNp is determined.

Further, in the second embodiment, the instructed shift position SRAMsftthat is stored in the nonvolatile memory is made a value that isdifferent from an initial value “0” that is set when power is applied tothe nonvolatile memory. More specifically, a value (e.g., 10)representing the P-position and a value (e.g., 20) representing thenon-P-position are stored in the nonvolatile memory. With this measure,at starting, whether the battery was removed can be determined on thebasis of whether the instructed shift position SRAMsft that is stored inthe nonvolatile memory is equal to the initial value “0.” Only routinesthat are different than in the first embodiment will be described.

[Setting of Instructed Shift Position]

An instructed shift position setting routine shown in FIG. 49 isactivated in a prescribed cycle (e.g., every 8 ms). Upon activation ofthis routine, at step 2901, it is determined whether the present instantbelongs to a starting period (i.e., the present instant is immediatelyafter resetting of the CPU of the ECU 41 or application of power to it).If it is determined that present instant does not belong to a startingperiod, the routine proceeds to step 2902, where it is determinedwhether the P-position switch 43 has been manipulated. If the P-positionswitch 43 has been manipulated, the routine proceeds to step 2903, wherethe instructed shift position sft is set to the P-position.

If the P-position switch 43 has not been manipulated, the routineproceeds to step 2904, where it is determined whether the non-P-positionswitch 44 has been manipulated. If the non-P-position switch 44 has beenmanipulated, the routine proceeds to step 2905, where the instructedshift position sft is set to the non-P-position. Then, the routineproceeds to step 2906, where the latest instructed shift position sft isstored in the nonvolatile memory of the ECU 41. Then this routine isfinished.

On the other hand, if it is determined at step 2901 that the presentinstant belongs to a starting period, the routine proceeds to step 2907,where whether the battery was removed is determined on the basis ofwhether the instructed shift position SRAMsft that is stored in thenonvolatile memory is equal to the initial value “0.” If it isdetermined that the battery was not removed, the routine proceeds tostep 2910, where the instructed shift position storage value SRAMsft ofthe nonvolatile memory is set as an instructed shift position sft ofstarting. Then this routine is finished.

If it is determined at step 2907 that the battery was removed, theroutine proceeds to step 2908, where a battery removal flag Xbat is setto “1” to perform a P-position-side butting control. At the next step2909, the instructed shift position storage value SRAMsft of thenonvolatile memory is set to the P-position. Then, the routine proceedsto step 2910, where the instructed shift position storage value SRAMsftof the nonvolatile memory is set as an instructed shift position sft ofstarting. Then this routine is finished.

[P-Position-Side Butting Control]

A P-position-side butting control routine shown in FIG. 50 is the sameas the first half (FIG. 39) of the P-position-side butting controlroutine of FIGS. 39 and 40 according to the first embodiment except thatstep 2711 is added to step 711 of the P-position-side butting controlroutine of FIGS. 39 and 40.

Upon activation of the P-position-side butting control routine of FIG.50, at step 2701, whether the battery was removed is determined on thebasis of whether the battery removal flag Xbat is “1.” If the batterywas not removed, step 2711 and the following steps are executed in thesame manner as in the first embodiment.

On the other hand, if it is determined that the battery removal flagXbat is “1” (i.e., the battery was removed), the routine proceeds tostep 2717, where the P-position-side butting control flag Xexp is set to“on.” A P-position-side butting control is performed thereafter in thesame manner as in the first embodiment, whereby a P-position-sidebutting learned value GNp is determined.

In the second embodiment described above, a latest instructed shiftposition sft is stored in the nonvolatile memory of the ECU 41 while thevehicle is driven. At starting, an instructed shift position storagevalue SRAMsft of the nonvolatile memory is set as an instructed shiftposition sft of starting. Therefore, even if the CPU of the ECU 41 isreset for a certain reason while the vehicle is driven, the instructedshaft range sft is not changed in association with the resetting, whichprevents trouble that the shift position is switched contrary to theintention of the driver.

Further, in the second embodiment, in view of the fact that aninstructed shift position storage value SRAMsft stored in thenonvolatile memory disappears upon removal of the battery, when removalof the battery is detected, the instructed shift position sft ofstarting is set to the P-position that is a shift position with whichwork of removing the battery is performed. This secures safety atstarting.

The nonvolatile memory may be a device such as an EEPROM that does notrequire a backup power source. In this case, an instructed shiftposition storage value can be held even if the battery is removed.Therefore, at starting it is appropriate to set an instructed shiftposition storage value of the nonvolatile memory as an instructed shiftposition of starting irrespective of whether the battery was removed. Itis not necessary to perform a P-position-side butting control again evenif the battery is removed.

Third Embodiment

During rotation of the SR motor 12, as shown in FIG. 52, the encoder 46outputs an A-phase signal pulse and a B-phase signal pulse alternatelyin synchronism with the rotation of the rotor 32 and also outputs aZ-phase signal pulse every time current supply is effected for all thephases around and the rotor 32 is rotated by 45°. Since referencerotation positions of the rotor 32 can be detected correctly by usingthe Z-phase signal, whether the corresponding relationship between therotation position of the rotor 32 and the current supply phase (i.e.,encoder count) is not deviated from a correct one can be checked bydetermining whether a current supply phase (i.e., encoder count) withwhich a Z-phase signal pulse is output corresponds to a referencerotation position of the rotor 32. A highly reliable motor control canbe performed by performing, if a deviation from the correct relationshipis found, a Z-phase correction for correcting for the deviation.

The Z-phase correction is performed by a Z-phase correction routineshown in FIG. 51. This routine is activated by A-phase interruptionprocessing in synchronism with both of rising edges and trailing edgesof an A-phase signal. Upon activation of this routine, at step 3401, itis determined whether a Z-phase signal value Z is equal to “1” (highlevel) and an encoder failure flag Xfail is “off” meaning that theencoder 46 is not in failure. If the Z-phase signal value Z is equal to“0” (low level) or the encoder failure flag Xfail is “on” meaning thatthe encoder 46 is in failure, the determination result at step 3401 is“no,” whereupon this routine is finished without executing the remainingsteps.

On the other hand, if the determination result at step 3401 is “yes,”the routine proceeds to step 3402, where a position detection count Nzonat the time of output of a Z-phase signal pulse (i.e., when the Z-phasesignal value Z has been inverted to “1”) is stored. The positiondetection count Nzon is an encoder count Ncnt at the time of output of aZ-phase signal pulse as corrected by using an initial positionaldeviation learned value Gcnt, and a reference rotation position (i.e., aposition where the Z-phase signal pulse is output) of the rotor 32 isdetected on the basis of the position detection count Nzon:Nzon=Ncnt−Gcnt

Then, the routine proceeds to step 4303, where whether the rotationdirection of the rotor 32 is the normal direction (i.e., the rotationdirection from the P-position to the non-P-position) on the basis ofwhether the instant of activation of this routine coincides with a riseof the A-phase signal.

As shown in FIG. 52, an A-phase signal pulse and a Z-phase signal pulseare output with A-phase difference of 90° (electrical angle). Therefore,in the case of the normal rotation direction (i.e., the rotationdirection from the P-position to the non-P-position), the A-phase signalrises during output of a Z-phase signal pulse. In case of the reverserotation direction (i.e., the rotation direction from the non-P-positionto the P-position), the A-phase signal falls during output of a Z-phasesignal pulse. Therefore, whether the current rotation direction is thenormal direction or the reverse direction can be determined on the basisof whether the A-phase signal rises or falls.

If it is determined at step 3403 that the instant of activation of thisroutine coincides with a rise of the A-phase signal, that is, therotation direction of the rotor 32 is the normal direction (i.e., therotation direction from the P-position to the non-P-position), theroutine proceeds to step 3404, where a deviation gz between an actualvalue and a design value of the current supply phase at the time ofoutput of the Z-phase signal pulse.gz=K1−mod(Nzon/12)

In the above equation, K1 is A-phase lead of the current supply phase inthe normal rotation direction that is necessary to rotate the rotor 32in the normal direction and is set to “4,” for example. And mod(Nzon/12)is a remainder obtained by dividing a position detection count Nzon withwhich the Z-phase signal pulse is output by 12.

In this embodiment, as shown in FIG. 52, the system is designed in sucha manner that mod(Nzon/12) becomes “4”, in the case of the normalrotation direction (i.e., the rotation direction from the P-position tothe non-P-position). Therefore, as long as the control system operatesnormally, gz=K1−mod(Nzon/12) becomes “0.”

On the other hand, if it is determined at step 3403 that the instant ofactivation of this routine coincides with a fall of the A-phase signal,that is, the rotation direction of the rotor 32 is the reverse direction(i.e., the rotation direction from then on-P-position to theP-position), the routine proceeds to step 3405, where a deviation gzbetween an actual value and a design value of the current supply phaseat the time of output of the Z-phase signal pulse.gz=K2−mod(Nzon/12)where K2 is A-phase lead of the current supply phase in the reverserotation direction that is necessary to rotate the rotor 32 in thereverse direction and is set to “3,” for example.

In this embodiment, as shown in FIG. 52, the system is designed in sucha manner that mod(Nzon/12) becomes “3” in the case of the reverserotation direction (i.e., the rotation direction from the non-P-positionto the P-position). Therefore, as long as the control system operatesnormally, gz=K2−mod(Nzon/12) becomes “0.”

After the calculation of the deviation gz between the actual value andthe design value of the current supply phase, the routine proceeds tostep 3406, where the deviation gz between the actual value and thedesign value is equal to “0.” If the deviation gz is equal to “0,” whichmeans that the control system is operating normally, thus routine isfinished without executing the remaining steps such as a Z-phasecorrection step.

On the other hand, if the deviation gz between the actual value and thedesign value of the current supply phase is not equal to “0,” it isdetermined that a Z-phase correction is necessary and the routineproceeds to step 3407. At step 3407, the count Cgz of anumber-of-Z-phase-corrections counter that counts the number of times ofZ-phase correction is incremented. The routine then proceeds to step3408, where a deviation of the initial positional deviation learnedvalue Gcnt is corrected for by the deviation gz between the actual valueand the design value.Gcnt=Gcnt−gz

Then, the routine proceeds to step 3409, where it is determined whetherthe count Cgz (i.e., the number of times of Z-phase correction) of thenumber-of-Z-phase-corrections counter has exceeded a determinationvalue. If the count Cgz is smaller than or equal to the determinationvalue, it is not determined yet that the encoder 46 is in failure. Onthe other hand, if the count Cgz is greater than the determinationvalue, it is determined that the encoder 46 is in failure and theroutine proceeds to step 3410. At step 3410, the encoder failure flagXfail is set to “on” meaning that the encoder 46 is in failure. Theroutine is then finished.

Once the encoder failure flag Xfail is set to “on,” the determinationresult at step 3401 is always “no” even if this routine is activated byA-phase interruption processing. Therefore, this routine is finishedforcibly and hence a Z-phase correction is not performed.

In this embodiment, whether the corresponding relationship between therotation position of the rotor 32 and the current supply phase (i.e.,encoder count) is not deviated from a correct one can be checked bydetermining whether a current supply phase (i.e., encoder count) withwhich a Z-phase signal pulse is output from the encoder 46 correspondsto A-phase corresponding to a reference rotation position of the rotor32, and, if a deviation from the correct relationship is found, aZ-phase correction is performed to correct for the deviation. A highlyreliable motor control can thus be performed.

If Z-phase corrections are performed repeatedly, there is a possibilitythat erroneous Z-phase corrections are being performed. In view of this,in this embodiment, if Z-phase corrections are performed a prescribednumber of times during an ordinary drive, further Z-phase correctionsare prohibited. This makes it possible to prevent the SR motor 12 frombeing stopped or operating erroneously due to repetition of erroneousZ-phase corrections.

In the Z-phase correction routine of FIG. 51, it is determined whether acorresponding relationship between a current supply phase (i.e., encodercount) and a rotation position of the rotor 32 when a Z-phase signalpulse is output from the encoder 46 is not deviated from a correct one.For example, whether a corresponding relationship between a currentsupply phase (i.e., encoder count) and a rotation position of the rotor32 is not deviated from a correct one may be determined on the basis ofwhether an increase or decrease of the encoder count from output of apreceding Z-phase signal pulse to output of a current Z-phase signalpulse is equal to “12.”

In the invention, an encoder that does not output a Z-phase signal(i.e., an encoder that outputs only an A-phase signal and a B-phasesignal may be employed. In this case, the Z-phase correcting function isemitted.

Fourth Embodiment

In the above embodiments, the phase lead of the current supply phase iscorrected in accordance with the rotation speed of the rotor 32 orpresence/absence of a deceleration request. However, almost the sameadvantages can be obtained by switching the current supply method inaccordance with the rotation speed (i.e., whether the present instantbelongs to an initial drive period) of the rotor 32 or presence/absenceof a deceleration request.

For example, if the one/two-phase current supply method is employed inan initial drive period, the rotor rotation angle per step (i.e., percurrent supply) is made a half of that of the one-phase current supplymethod in which current supply is always effected for only one phase andthe two-phase current supply method in which current supply is alwayseffected for two phases, whereby rotation of the rotor 32 can be startedquickly by virtue of high torque. If the rotor 32 is thereafter drivenby the one-phase current supply method in which current supply is alwayseffected for only one phase, the rotor rotation angle per step (i.e.,per current supply) is made two times that in the initial drive period(one/two-phase current supply method), whereby the rotor 32 can berotated stably at high speed.

In this case, when it becomes necessary to decelerate the rotation speedof the rotor 32 (e.g., when the rotor 32 comes close to a targetposition) in a period when the rotor 32 is being driven by the one-phasecurrent supply method, switching may be made from the one-phase currentsupply method to the one/two-phase current supply method. That is, ifswitching is made to the one/two-phase current supply method in a statethat the rotor 32 is being rotated by the one-phase current supplymethod, the phase of the current supply phase delays from the actualrotation phase of the rotor 32, whereby torque (braking torque) forrotating the rotor 32 in the direction opposite to the current rotationdirection is generated. The rotation speed of the rotor 32 can bedecreased reliably. In addition, in the one/two-phase current supplymethod, the rotor rotation speed per step (i.e., per current supply) isa half of that of the one-phase current supply method. Therefore, therotor 32 can accurately be stopped at a target position.

The fourth embodiment of the invention which is an implementation of theabove concept will be described with reference to FIGS. 53 and 54.

A current supply method setting routine shown in FIG. 53, which isactivated in a prescribed cycle (e.g., every 1 ms), sets a currentsupply method in the following manner in accordance with a rotationspeed of the rotor 32 (i.e., whether the present instant belongs to aninitial drive period) or presence/absence of a deceleration request.Upon activation of this routine, at step 4701, it is determined whetherthe feedback permission flag Xfb is “on” (a feedback control is beingperformed). If the feedback permission flag Xfb is “off” (feedbackcontrol prohibited), this routine is finished without executing theremaining steps.

On the other hand, if the feedback permission flag Xfb is “on” (afeedback control is being performed), the routine proceeds to step 4702,where it is determined whether the present instant belongs to an initialdrive period is determined by using at least one of the following threecriteria:

(1) Whether the rotation speed SP of the rotor 32 is lower than or equalto a prescribed value.

(2) Whether the variation of the encoder count (i.e., rotor rotationamount) from the start of the drive is smaller than or equal to aprescribed value.

(3) Whether the elapsed time from the start of the drive is shorter thanor equal to a prescribed value.

Whether the present instant belongs to an initial drive period may bedetermined on the basis of whether two or more of the above criteria(1)-(3) are met. Naturally, a criterion other than the above criteria(1)-(3) may be used. For example, whether the present instant belongs toan initial drive period may be determined on the basis of whether themotor load torque is higher than or equal to a prescribed value.

If it is determined at step 4702 that the present instant belongs to aninitial drive period, the routine proceeds to step 4703, where thecurrent supply method is set to the one/two-phase current supply method.At the next step 4704, the speed phase lead correction amount Ks is setto “0,” for example. As a result, in the initial drive period, the rotor32 is driven by the one/two-phase current supply method as in the caseof the above embodiments.

On the other hand, if it is determined at step 4702 that the presentinstant does not belong to an initial drive period, the routine proceedsto step 4705, where it is determined whether a deceleration request hasoccurred. For example, whether the rotation position of the rotor 32 hascome close to a target position (i.e., whether the rotation position ofthe rotor 32 has entered a deceleration range for a stop) is determinedon the basis of whether the absolute value |Acnt−Ncnt| of the differencebetween the target count Acnt and the encoder count Ncnt is smaller thana prescribed value. Or whether the rotation position of the rotor 32 hasentered a deceleration range that is set in a feedback control.

If no deceleration request has occurred, the routine proceeds to step4706, where the current supply method is set to the one-phase currentsupply method. In the one-phase current supply method, a current supplyphase is determined on the basis of a value Mptn %12 by using aconversion table shown in FIG. 54.

When a deceleration request occurs afterward, the determination resultat step 4705 becomes “yes” and the routine proceeds to step 4707. Atstep 4707, the current supply method is set to the one/two-phase currentsupply method. At the next step 4708, the speed phase lead correctionamount Ks is set to “−1,” for example. As a result, torque (brakingtorque) for rotating the rotor 32 in the direction opposite to thecurrent rotation direction is generated. The rotation speed of the rotor32 can be decreased reliably.

Combining the switching of the current supply method and the correctionof the phase lead of the current supply phase in the above manner makesit possible to increase the starting performance, high-speed rotationperformance, and the stop performance (deceleration performance)effectively. However, in the invention, only the switching of thecurrent supply method may be employed (the correction of the phase leadof the current supply phase) is omitted.

Further, the rotation speed of the rotor 32 may be controlled inaccordance with a request rotation speed (i.e., a target rotation speed)by setting a request rotation speed (i.e., a target rotation speed)during a feedback control and correcting the phase lead of the currentsupply phase or switching the current supply method in accordance withthe thus-set request rotation speed (i.e., target rotation speed). Forexample, when it is intended to increase the rotation speed of the rotor32, it is appropriate to correct the phase lead of the current supplyphase so that it is increased or to switch the current supply methodfrom the one/two-phase current supply method to the one-phase currentsupply method (or two-phase current supply method). Conversely, when itis intended to decrease the rotation speed of the rotor 32, it isappropriate to correct the phase lead of the current supply phase sothat it is decreased or to switch the current supply method from theone-phase current supply method (or two-phase current supply method) tothe one/two-phase current supply method. In this manner, the rotationspeed of the rotor 32 can be controlled while a feedback control isperformed on the basis of the encoder count so that the rotor 32 isrotated to a target position.

The position switching device according to this embodiment is such thatswitching is made between the two ranges, that is, the P-position andthe non-P-position. However, for example, the invention can also beapplied to a position switching device that switches between ranges ofP, R, N, D, etc. of an automatic transmission by switching between aposition switching valve and a manual valve of the automatictransmission in link with a rotation operation of the detent lever 15.

Fifth Embodiment

Where a position switching control is performed with the SR motor 12having the encoder 46, a feedback control for rotating the rotor 32 to atarget position is performed every time the instructed shift position(target position) is switched from the P-position to the non-P-positionor in the opposite direction by rotating the rotor 32 by sequentiallyswitching the current supply phase of the rotor 12 on the basis of theencoder count. When the encoder count has reached a target count that isset in accordance with the target position, it is determined that therotor 32 has reached the target position and the feedback control isfinished, whereby the rotor 32 is stopped at the target position.

In a feedback control on the SR motor 12, the current supply phase isswitched in synchronism with output of A-phase signal pulses and B-phasesignal pulses from the encoder 46. Therefore, if rotation of the rotor32 is stopped for a certain reason during a feedback control the encoder46 stops outputting the A-phase signal and the B-phase signal, thecurrent supply phase cannot be switched in the feedback control andhence the rotor 32 cannot be rotated to a target position.

In view of the above, in a fifth embodiment of the invention, the ECU 41of the position switching controller 42 executes, parallel with afeedback control on the SR motor 12, in the following manner, atime-synchronous current supply phase setting process that sets thecurrent supply phase on the basis of the encoder count in a prescribedcycle (e.g., every 1 ms) from a start of the feedback control until therotor 32 is rotated to a rotation position.

With this configuration, even if the rotation of the rotor 32 is oncestopped for a certain reason during a feedback control and the encoder46 stops outputting the A-phase signal and the B-phase signal, thetime-synchronous current supply phase setting process sets the currentsupply phase on the basis of an encoder count at that instant.Therefore, the time-synchronous current supply phase setting process canswitch the current supply phase and hence rotate the rotor 32 so that itcomes as close to a target position as possible. The reliability of thedrive control (position switching control) on the SR motor 12 can thusbe increased.

[Time-Synchronous Motor Control]

A time-synchronous motor control routine shown in FIG. 55 is activatedin a prescribed cycle (e.g., every 1 ms) after the end of an initialdrive. An ordinary drive (a feedback control start position stopping andholding process, a time-synchronous current supply phase settingprocess, and a feedback control), a target position stopping and holdingprocessing, or a reversing position stopping and holding process isexecuted in this routine.

Upon activation of this routine, at step 5501, it is determined whetherthe control mode determination value “mode” is equal to “1” (ordinarydrive). If the control mode determination value “mode” is equal to “1,”the routine proceeds to step 5505, where a mode-1 routine (describedlater) shown in FIG. 56 is executed, whereby a current supply phasedetermination value Mptn indicating a current supply phase with which toexecute a feedback control start position stopping and holding processand a time-synchronous current supply phase setting process iscalculated.

If it is determined at step 5501 that the control mode determinationvalue “mode” is not equal to “1,” the routine proceeds to step 5502,where it is determined whether the feedback permission flag Xfb is “off”(feedback control prohibited). If the feedback permission flag Xfb is“on” (feedback control permitted), this routine is finished withoutexecuting the remaining steps. In this case, current supply phasesetting and current supply processing are performed by a feedbackroutine (see FIG. 30).

In this routine, if the control mode determination value “mode” is notequal to “1,” step 5505 (i.e., the mode-1 routine of FIG. 56) isexecuted even during a feedback control. That is, the feedback controlin which current supply phases are set in synchronism with output ofA-phase signal pulses and B-phase signal pulses from the encoder 46 bythe feedback control routine of FIG. 30 and a time-synchronous currentsupply phase setting process in which current supply phases are set in aprescribed cycle by the mode-1 routine are executed in parallel. Withthis measure, even if the rotor 32 is stopped for a certain reasonduring a feedback control, current supply phase determination valuesMptn are calculated by the time-synchronous current supply phase settingprocess and the rotor 32 is rotated toward a target position.

On the other hand, if it is determined at step 5502 that the feedbackcontrol permission flag Xfb is “off (feedback control prohibited), it isdetermined at steps 5503 and 5504 whether the control mode determinationvalue “mode” is equal to 3 or 4. If the control mode determination value“mode” is equal to “3” (target position stopping and holding process),the routine goes from step 5503 to step 5506, where the mode-3 routineshown in FIG. 25 is executed, whereby a current supply phasedetermination value Mptn indicating a current supply phase with which toexecute a target position stopping and holding process is calculated.

If the control mode determination value “mode” is equal to “4”(reversing position stopping and holding process), the routine goes fromstep 5504 to step 5507, where a mode-4 routine shown in FIG. 26 isexecuted, whereby a current supply phase determination value Mptnindicating a current supply phase with which to execute a current supplyposition stopping and holding process is calculated.

As described above, if the control mode determination value “mode” isequal to 1, 3, or 4, a current supply phase determination value Mptn iscalculated and the routine proceeds to step 5508, where a current supplyprocessing routine shown in FIG. 27 is executed, that is, an ordinarydrive, a target position stopping and holding processing, or a reversingposition stopping and holding process is executed.

On the other hand, if the determination results at both of steps 5503and 5504 are “no,” that is, if the control mode determination value“mode” is equal to 0 or 5, the routine directly proceeds to step 5508,where the current supply processing routine shown in FIG. 27 isexecuted, that is, the current supply is turned off or an open-loopcontrol is performed.

[Mode-1]

The mode-1 routine shown in FIG. 56 is a subroutine that is activated atstep 5505 of the time-synchronous motor control routine of FIG. 55. Inthe mode-1 routine, a current supply phase determination value Mptn(current supply phase) for a feedback control start position stoppingand holding process and a time-synchronous current supply phase settingprocess is set in the following manner.

Upon activation of this routine, at step 5511, the count CT1 of thecurrent supply time counter that counts the time of a feedback controlstart position stopping and holding process is incremented. At the nextstep 5512, it is determined whether the time CT1 of the feedback controlstart position stopping and holding process has exceeded a prescribedtime (e.g., 10 ms).

If the time CT1 of the feedback control start position stopping andholding process has not exceeded the prescribed time (e.g., 10 ms), theroutine proceeds to step 5513, where it is determined whether thestopping and holding current supply phase storage flag Xhold is “off”(not stored), that is, whether the present instant is immediately beforea start of the feedback control start position stopping and holdingprocess. If the stopping and holding current supply phase storage flagXhold is “off,” the routine proceeds to step 5514, where a currentposition count Ncnt−Gcnt is set as a current supply phase determinationvalue Mptn for the feedback control start position stopping and holdingprocess:Mptn=Ncnt−Gcnt

The current position count Ncnt−Gcnt is an encoder count Ncnt ascorrected by using an initial positional deviation learned value Gcntthat was learned in an initial drive, and represents a current positionof the rotor 32 correctly.

Then, the routine proceeds to step 5515, where the current supply phasedetermination value Mptn is divided by 12 and a remainder Mptn %12 isobtained. The number “12” is an increase or decrease of the encodercount Ncnt (current supply phase determination value Mptn) that occurswhen current supply is effected for all the phases around. A currentsupply phase is determined according to the conversion table shown inFIG. 28 on the basis of the value Mptn %12.

At step 5516, whether one-phase current supply (U-phase, V-phase, orW-phase) is going to be performed is determined on the basis of whetherthe value Mptn %12 is equal to 2, 3, 6, 7, 10, or 11. If one-phasecurrent supply is going to be performed, the routine proceeds to step5517, where the current supply phase determination value Mptn isincremented by “2” that corresponds to one step so that two-phasecurrent supply (U and V-phases, V and W-phases, or U and W-phases) willbe performed. Executing the feedback control start position stopping andholding process with two-phase current supply which produces highertorque than one-phase current supply does prevents the rotor 32 fromvibrating in the vicinity of a feedback control start position and hencecan reliably stop and keep the rotor 32 at the feedback control startposition.

The stopping and holding current supply phase storage flag Xhold is setto “on” (stored) at the next step 518, Then this routine is finished.When this routine is activated later, the determination result at step5513 should become “no” and hence steps 5514-5518 are not executed. Thatis, the process of setting a current supply phase determination valueMptn (current supply phase) for the feedback control start positionstopping and holding process is set is performed only once immediatelybefore a start of the feedback control start position stopping andholding process.

Then, when the time CT1 of the feedback control start position stoppingand holding process has exceeded the prescribed time (e.g., 10 ms), thedetermination result at step 5512 becomes “yes,” whereupon the feedbackcontrol start position stopping and holding process is finished and atransition is made to a feedback control. During the feedback control,every time this routine is activated in the prescribed cycle (e.g.,every 1 ms), the current supply phase setting routine of FIG. 31 isexecuted at step 5519, whereby a current supply phase determinationvalue Mptn is calculated. It is noted that the current supply phasesetting routine is also activated by a feedback control routine. Then,the routine proceeds to step 5520, where the feedback permission flagXfb is set to “on” (feedback control permitted).

In the control mode setting routine of FIGS. 20-22, when the differenceAcnt−Ncnt between the target count Acnt and the encoder count Ncnt hasbecome smaller than or equal to the prescribed value because of theexecution of the feedback control, it is determined that the rotor 32has reached the target position (timing for finishing the feedbackcontrol) and the feedback permission flag Xfb is set to “off.” Thefeedback control is finished and the control mode determination value“mode” is set to “3” (target position stopping and holding process).When the prescribed time (e.g., 50 ms) has elapsed thereafter, thecontrol mode determination value “mode” is set to “0” (current supplyoff).

Since the mode-1 routine of FIG. 56 is not activated after the end ofthe feedback control, the current supply phase setting by thetime-synchronous current supply phase setting process (step 5519) isperformed from the start of the feedback control until the rotor 32reaches the target position (i.e., until the feedback control isfinished).

FIG. 32 is a time chart illustrating A-phase for which current supply iseffected first in the case where rotation is started from a statecorresponding to the U and W-phases. In this case, to start normalrotation (i.e., rotation from the P-position to the non-P-position), acurrent supply phase determination value Mptn is calculated according tothe following equation by using an encoder count Ncnt, an initialpositional deviation learned value Gcnt, and a normal rotation directionphase lead K1:Mptn=Ncnt−Gcnt+K1

If the normal rotation direction phase lead K1 is equal to 4, forexample, a current supply phase determination value Mptn is calculatedaccording to the following equation:Mptn=Ncnt−Gcnt+4To start normal rotation from a state corresponding to the U andW-phases, mod(Ncnt−Gcnt) is 4 and hence Mptn %12 is equal to 8 (=4+4).The U and V-phases are set as first current supply phases.

On the other hand, to start reverse rotation (i.e., rotation from thenon-P-position to the P-position) from a state corresponding to the Uand W-phases, if a reverse rotation direction phase lead K2 is equal to3, for example, a current supply phase determination value Mptn iscalculated according to the following equation:

Mptn = Ncnt − Gcnt − K 2     = Ncnt − Gcnt − 3To start reverse rotation from a state corresponding to the U andW-phases, Mptn %12 is equal to 1 (=4−3). The V and W-phases are set asfirst current supply phases.

In this manner, by setting the normal rotation direction phase lead K1and the reverse rotation direction phase lead K2 to 4 and 3,respectively, current supply phase switching patterns for the normalrotation direction and the reverse rotation direction can be madesymmetrical. In either of the normal rotation direction and the reverserotation direction, rotation can be started by effecting current supplyfirst for A-phase of a position that is two steps deviated from thecurrent position of the rotor 32.

[Current Supply Phase Setting]

The current supply phase setting routine of this embodiment is the sameas that shown in FIG. 31. Upon activation of this routine, at step 611,it is determined whether the rotation direction instruction value Dindicating a rotation direction toward a target position is equal to “1”that means the normal rotation direction (i.e., the rotation directionfrom the P-position to the non-P-position). If it is determined that therotation direction instruction value D is equal to “1” (normalrotation), the routine proceeds to step 612, where it is determinedwhether the rotation direction of the rotor 32 has reversed (i.e.,whether the encoder count Ncnt has decreased) contrary to the rotationdirection instruction. If the rotation direction has not reversed, theroutine proceeds to step 613, where the current supply phasedetermination value Mptn is updated according to the following equationby using a current encoder count Ncnt, an initial positional deviationlearned value Gcnt, a normal rotation direction phase lead K1, and aspeed phase lead correction amount Ks:Mptn=Ncnt−Gcnt+K1+Ks

The normal rotation direction phase lead K1 is A-phase lead of thecurrent supply phase that is necessary to rotate the rotor 32 in thenormal direction (i.e., A-phase lead of the current supply phase withrespect to the current rotation phase of the rotor 32), and is set to“4,” for example.

The speed phase lead correction amount Ks is A-phase lead correctionamount that is set in accordance with a rotation speed of the rotor 32.For example, the speed phase lead correction amount Ks is set to “0” ina low-speed range and is increased to “1” or “2” as the rotation speedincreases. As a result, the current supply phase determination valueMptn is corrected so as to indicate a current supply phase that issuitable for a rotation speed of the rotor 32.

On the other hand, if it is determined at step 612 that the rotationdirection of the rotor 32 has reversed contrary to the rotationdirection instruction, the current supply phase determination value Mptnis not updated to prevent progress of the reversed rotation. In thiscase, current supply is effected for the phase for which current supplywas effected immediately before the reversing (i.e., the precedingcurrent supply phase), whereby braking torque is generated in such adirection as to suppress the reversed rotation.

If it is determined at step 611 that the rotation direction instructionvalue D is equal to “−1” (the reverse rotation direction, i.e., therotation direction from the non-P-position to the P-position), theroutine proceeds to step 614, where it is determined whether therotation direction of the rotor 32 has reversed (i.e., whether theencoder count Ncnt has increased) contrary to the rotation directioninstruction. If the rotation direction has not reversed, the routineproceeds to step 615, where the current supply phase determination valueMptn is updated according to the following equation by using a currentencoder count Ncnt, an initial positional deviation learned value Gcnt,a reverse rotation direction phase lead K2, and a speed phase leadcorrection amount Ks:Mptn=Ncnt−Gcnt−K2−Ks

The reverse rotation direction phase lead K2 is A-phase lead of thecurrent supply phase that is necessary to rotate the rotor 32 in thereverse direction (i.e., A-phase lead of the current supply phase withrespect to the current rotation phase of the rotor 32), and is set to“3,” for example. The speed phase lead correction amount Ks is set inthe same manner as in the case of the normal rotation.

On the other hand, if it is determined at step 614 that the rotationdirection of the rotor 32 has reversed contrary to the rotationdirection instruction, the current supply phase determination value Mptnis not updated to prevent progress of the reversed rotation (i.e.,normal rotation). In this case, current supply is effected for the phasefor which current supply was effected immediately before the reversing(i.e., the preceding current supply phase), whereby braking torque isgenerated in such a direction as to suppress the reversed rotation(i.e., normal rotation).

After the current supply phase determination value Mptn has beendetermined in the above manner, the current supply processing routine isexecuted. While the feedback control is being performed, a currentsupply phase corresponding to a value Mptn %12 is selected at step 555by searching the conversion table and the windings 33 or 34 of theselected current supply phase are energized.

In the above embodiment, a time-synchronous current supply phase settingprocess that sets the current supply phase on the basis of the encodercount in a prescribed cycle (e.g., every 1 ms) from a start of afeedback control on the SR motor 12 until the rotor 32 is rotated to arotation position is executed parallel with the feedback control.Therefore, even if the rotation of the rotor 32 is stopped for a certainreason during a feedback control the encoder 46 stops outputting anA-phase signal and a B-phase signal, the time-synchronous current supplyphase setting process can set a current supply phase on the basis of anencoder count at that instant. Therefore, the rotor 32 can be rotated soas to come as close to a target position as possible. The reliability ofthe drive control (position switching control) on the SR motor 12 canthus be increased.

In this embodiment, a time-synchronous current supply phase settingprocess is executed in the entire period from the start of a feedbackcontrol to an instant when the rotor 32 reaches a target position.Therefore, with whatever timing the rotor 32 is stopped, thetime-synchronous current supply phase setting process can set a currentsupply phase without delay immediately after the stop of the rotor 32.This provides an advantage that the stop time of the rotor 32 can beshortened.

However, in the invention, a time-synchronous current supply phasesetting process may be continued until the rotor 32 is rotated to atarget position in a period when the rotation speed of the rotor 32 islower than or equal to a prescribed value. With this measure, atime-synchronous current supply phase setting process is executed onlyafter the rotation speed of the rotor 32 has decreased to a value atwhich the rotor 32 may stop temporarily. This provides an advantage thatthe computation load of the CPU of the ECU 41 can be reduced.

As a further alternative, a time-synchronous current supply phasesetting process may be executed only when the rotor 32 has stopped inthe middle of a feedback control.

The present invention should not be limited to the above embodiments andmodifications, but may be implemented in various other ways withoutdeparting from the spirit of the invention.

1. A motor control apparatus comprising: an encoder for producing a pulse output signal in synchronism with rotation of a rotor of a motor; and control means for feedback-controlling the rotor to a target position by detecting a rotation position of the rotor on the basis of a count of the output signal of the encoder and sequentially switching a current supply phase of the motor, wherein the control means has a first correction part that corrects an amount of phase lead of the current supply phase with respect to a rotation phase of the rotor in accordance with a rotation speed of the rotor, and the control means further has a second correction part that corrects the amount of phase lead of the current supply phase in a direction to decrease relative to the amount of phase lead determined by the first correction part, when the rotation speed of the rotor is required to decrease.
 2. The motor control apparatus according to claim 1, wherein the motor is a switched reluctance motor.
 3. The motor control apparatus according to claim 1, wherein the motor drives a position switching mechanism for switching between positions of an automatic transmission of the vehicle.
 4. The motor control apparatus according to claim 1, wherein: the rotation speed of the rotor is a target rotation speed of the rotor, to which the control means feedback-controls the rotation of the rotor.
 5. The motor control apparatus according to claim 1, wherein: the control means further has a position determination part that compares the rotation position of the rotor with the target position, and determines that the rotation speed of the rotor is required to decrease when the rotation position of the rotor is within a predetermined range relative to the target position.
 6. A motor control apparatus comprising: an encoder for producing a pulse output signal in synchronism with rotation of a rotor of a motor; and control means for feedback-controlling the rotor to a target position by detecting a rotation position of the rotor on the basis of a count of the output signal of the encoder and sequentially switching a current supply phase of the motor, wherein the control means controls a rotation speed of the rotor by correcting an amount of phase lead of the current supply phase with respect to a rotation phase of the rotor and switching a current supply method in accordance with a rotation speed of the rotor, the control means has a determination part that determines whether a deceleration request is present, and the control means performs a one-phase current supply method, and a one/two-phase current supply method, when the determination part determines that the deceleration request is absent and present, respectively.
 7. The motor control apparatus according to claim 6, wherein: the position determination part that compares the rotation position of the rotor with the target position to determine that the deceleration request is present when the rotation position of the rotor is within a predetermined range relative to the target position.
 8. The motor control apparatus according to claim 7, wherein: the control means corrects the amount of phase lead only in case of performing the one/two-phase current supply method. 